diff --git a/E4-PropertyBasedTests b/E4-PropertyBasedTests
new file mode 120000
index 0000000..8c53209
--- /dev/null
+++ b/E4-PropertyBasedTests
@@ -0,0 +1 @@
+HeartRate
\ No newline at end of file
diff --git a/propertybasedtesting/.gitignore b/HeartRate/.gitignore
similarity index 100%
rename from propertybasedtesting/.gitignore
rename to HeartRate/.gitignore
diff --git a/HeartRate/.projectile b/HeartRate/.projectile
new file mode 100644
index 0000000..e69de29
diff --git a/propertybasedtesting/.scalafmt.conf b/HeartRate/.scalafmt.conf
similarity index 84%
rename from propertybasedtesting/.scalafmt.conf
rename to HeartRate/.scalafmt.conf
index bf2da0b..2321096 100644
--- a/propertybasedtesting/.scalafmt.conf
+++ b/HeartRate/.scalafmt.conf
@@ -1,3 +1,4 @@
+version = "2.0.0"
 style = defaultWithAlign
 maxColumn = 120
 project.git = true
diff --git a/propertybasedtesting/LICENSE b/HeartRate/LICENSE
similarity index 100%
rename from propertybasedtesting/LICENSE
rename to HeartRate/LICENSE
diff --git a/HeartRate/README.md b/HeartRate/README.md
new file mode 100644
index 0000000..422bdb1
--- /dev/null
+++ b/HeartRate/README.md
@@ -0,0 +1,26 @@
+# What If We exclusively use property based testing
+
+## Proposition
+
+  * Property based testing is an effective way of documenting contracts (pre and 
+  post conditions to functions), or other laws / properties in your codebase.
+  * Property based testing facilitates the generation of specific test cases and 
+  thus speeds up the speed with which you write tests.
+  * Property based testing reduces duplication, but providing a framework for 
+  re-use of specific data values.
+  * Property based testing increases the coverage of your code and thus helps 
+  detect harder to find bugs.
+
+## Experiment Videos
+
+### From unit to property
+Convert an existing unit test, to a property based test.
+
+### From property to unit.
+Use the generator framework to run a specific test.
+
+### Refine the input types.
+Use type refinement to ensure the compiler catches illegal values instead of relying on unit testing.
+
+### Refine dependent input.
+Use type refinements and path dependent types to ensure the compiler catches illegal values instead of relying on unit testing.
\ No newline at end of file
diff --git a/propertybasedtesting/build.sbt b/HeartRate/build.sbt
similarity index 96%
rename from propertybasedtesting/build.sbt
rename to HeartRate/build.sbt
index 8a619c1..6f82056 100644
--- a/propertybasedtesting/build.sbt
+++ b/HeartRate/build.sbt
@@ -7,10 +7,10 @@ lazy val root = (project in file("."))
     "-Ywarn-unused:imports",
     "-Xfatal-warnings"
   ))))
-//  .enablePlugins(ScalafmtPlugin)
+  .enablePlugins(ScalafmtPlugin)
 
 lazy val projectSettings = Seq(
-  name := "PropertyBasedTesting",
+  name := "HeartRateSample",
   version := "0.1.0",
   organization := "io.whatifs",
   scalaVersion := "2.12.8",
@@ -34,9 +34,9 @@ lazy val headerSettings = Seq(
 )
 
 lazy val projectDependencies = Seq(
-compilerPlugin("org.typelevel" %% "kind-projector" % "0.10.3"),
-
-"org.scalatest" %% "scalatest" % "3.0.8" % "test",
+  compilerPlugin("org.typelevel" %% "kind-projector" % "0.10.3"),
+  "eu.timepit" %% "refined" % "0.9.8",
+  "org.scalatest" %% "scalatest" % "3.0.8" % "test",
   "org.scalacheck" %% "scalacheck" % "1.14.0" % "test"
 )
 
diff --git a/HeartRate/notes.md b/HeartRate/notes.md
new file mode 100644
index 0000000..26de8dd
--- /dev/null
+++ b/HeartRate/notes.md
@@ -0,0 +1,19 @@
+# Notes
+
+## Type Refinement Video Title Ideas
+
+* What if your compiler was your proof assistant
+* What if your compiler can predict your runtime errors
+* What if your compiler was your static analysis
+
+## Alternative Languages
+
+* Think about duplicating the HeartRates code in C# and javascript.
+
+## Dramatic oneliner snippets
+
+Also think about more dramatic shorter videos with close to 0 domain knowledge requirements.
+
+### Headline Ideas
+
+* Lines of Code that will Change Your Life
diff --git a/propertybasedtesting/project/build.properties b/HeartRate/project/build.properties
similarity index 100%
rename from propertybasedtesting/project/build.properties
rename to HeartRate/project/build.properties
diff --git a/HeartRate/project/plugins.sbt b/HeartRate/project/plugins.sbt
new file mode 100644
index 0000000..2bf429c
--- /dev/null
+++ b/HeartRate/project/plugins.sbt
@@ -0,0 +1,5 @@
+logLevel := Level.Warn
+
+resolvers += Resolver.sonatypeRepo("releases")
+
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.0.0")
diff --git a/HeartRate/src/main/scala/ArchivalMeassurement.scala b/HeartRate/src/main/scala/ArchivalMeassurement.scala
new file mode 100644
index 0000000..3da2441
--- /dev/null
+++ b/HeartRate/src/main/scala/ArchivalMeassurement.scala
@@ -0,0 +1,37 @@
+package io.whatifs.heartrates
+
+import eu.timepit.refined._, api.{Refined, Validate}
+
+import java.time.LocalDate
+
+case class ArchivalDatePredicate()
+case class ArchivalPredicate[DT](dt: DT)
+
+object ArchivalDatePredicate {
+    implicit def archivalDatePredicateValidate: Validate.Plain[LocalDate, ArchivalDatePredicate] = 
+        Validate.fromPartial(_.isBefore(LocalDate.now()), "ArchivalDate", ArchivalDatePredicate())
+}
+
+object ArchivalPredicate {    
+    implicit def archivalDateValidate[DT <: LocalDate](implicit ws: W.Aux[DT]): Validate.Plain[Measurement, ArchivalPredicate[DT]] = 
+        Validate.fromPredicate(m => !m.date.isAfter(ws.value), t => s"""$t.archivalDate(${ws.value})""", ArchivalPredicate(ws.value))
+}
+
+case class ArchivedMeasurement private(measurement: Measurement, archiveDate: ArchivedMeasurement.ArchivalDate) {
+
+    def this(archiveDate: ArchivedMeasurement.ArchivalDate)(
+        measurement: ArchivedMeasurement.ArchivalMeasurement[archiveDate.value.type]) =
+        this(measurement.value, archiveDate)
+}
+
+object ArchivedMeasurement {
+
+    type ArchivalDate = LocalDate Refined ArchivalDatePredicate
+    type ArchivalMeasurement[DT] = Measurement Refined ArchivalPredicate[DT]
+    
+    def archivedMeasurement(archiveDate: LocalDate, measurement: Measurement): Either[String, ArchivedMeasurement] = for {
+        dt <- refineV[ArchivalDatePredicate](archiveDate)
+        m  <- refineV[ArchivalPredicate[dt.value.type]](measurement)
+    } yield new ArchivedMeasurement(dt)(m)
+
+}
diff --git a/HeartRate/src/main/scala/HeartRate.scala b/HeartRate/src/main/scala/HeartRate.scala
new file mode 100644
index 0000000..432a477
--- /dev/null
+++ b/HeartRate/src/main/scala/HeartRate.scala
@@ -0,0 +1,21 @@
+package io.whatifs.heartrates
+
+import eu.timepit.refined.{W, refineV}
+import eu.timepit.refined.api.Refined
+import eu.timepit.refined.numeric.Interval
+
+case class HeartRate(bpm: Int Refined HeartRate.ValidHeartRate) {
+  def isHigh = bpm.value < 100
+}
+object HeartRate {
+  final val MinHeartRate = 60
+  final val MaxHeartRate = 300
+
+  type ValidHeartRate = Interval.Closed[W.`MinHeartRate`.T, W.`MaxHeartRate`.T]
+
+  def build(bpm: Int): Either[String, HeartRate] = {
+    refineV[ValidHeartRate](bpm)
+      .map(HeartRate.apply)
+      .left.map(_ => "Illegal heart rate")
+  }
+}
diff --git a/HeartRate/src/main/scala/Meassurement.scala b/HeartRate/src/main/scala/Meassurement.scala
new file mode 100644
index 0000000..0a961a5
--- /dev/null
+++ b/HeartRate/src/main/scala/Meassurement.scala
@@ -0,0 +1,25 @@
+package io.whatifs.heartrates
+
+import java.time.LocalDate
+
+case class Person(name: String, age: Int)
+
+case class Measurement(person: Person, heartRate: HeartRate, date: LocalDate) {
+  import Measurement._
+  def warn: Option[String] =
+    if (heartRate.isHigh && date.isAfter(warnIfAfter))
+      Some("Heart Rate is High")
+    else None
+
+}
+
+object Measurement {
+  val WarningDays = 3
+
+  def measure(person: Person, heartRate: HeartRate): Measurement = {
+    new Measurement(person, heartRate, LocalDate.now())
+  }
+
+  def warnIfAfter: LocalDate =
+    LocalDate.now().minusDays(WarningDays.toLong)
+}
diff --git a/HeartRate/src/test/scala/ArchivalMeassurementSpec.scala b/HeartRate/src/test/scala/ArchivalMeassurementSpec.scala
new file mode 100644
index 0000000..d6388f1
--- /dev/null
+++ b/HeartRate/src/test/scala/ArchivalMeassurementSpec.scala
@@ -0,0 +1,23 @@
+package io.whatifs.heartrates
+
+import org.scalatest.{Matchers, PropSpec}
+import org.scalatestplus.scalacheck.ScalaCheckDrivenPropertyChecks
+
+import java.time.LocalDate
+
+class ArchivedMeasurementSpec
+    extends PropSpec
+    with ScalaCheckDrivenPropertyChecks
+    with Matchers {
+        import HeartRateGenerators._
+
+property ("Measurements can be archived according to the right rules") {
+    forAll (dateGen, measurementGen) { 
+        case (archiveDate, measurement) if !archiveDate.isBefore(measurement.date) && archiveDate.isBefore(LocalDate.now()) => 
+      ArchivedMeasurement.archivedMeasurement(archiveDate, measurement).isRight
+      case (archiveDate, measurement) => 
+      ArchivedMeasurement.archivedMeasurement(archiveDate, measurement).isLeft
+  }
+}
+
+    }
\ No newline at end of file
diff --git a/HeartRate/src/test/scala/HearRateGenerators.scala b/HeartRate/src/test/scala/HearRateGenerators.scala
new file mode 100644
index 0000000..bc8c40f
--- /dev/null
+++ b/HeartRate/src/test/scala/HearRateGenerators.scala
@@ -0,0 +1,35 @@
+package io.whatifs.heartrates
+
+import java.time.LocalDate
+
+import org.scalacheck.Gen
+
+import eu.timepit.refined.refineV
+
+object HeartRateGenerators {
+  val specialBPMValues: Seq[Int] =
+    Seq(HeartRate.MinHeartRate, HeartRate.MaxHeartRate)
+      .flatMap(bpm => Seq(bpm - 1, bpm, bpm + 1))
+
+  val bpmGen: Gen[Int] = Gen.chooseNum(Int.MinValue, Int.MaxValue, specialBPMValues:_*).label("bpm")
+
+  val hrGen: Gen[HeartRate] = Gen.chooseNum(HeartRate.MinHeartRate, HeartRate.MaxHeartRate)
+  .map(refineV[HeartRate.ValidHeartRate](_))
+  .flatMap(_.fold(_ => Gen.fail, bpm => Gen.const(HeartRate.apply(bpm))))
+
+  val personGen: Gen[Person] = for {
+    name <- Gen.alphaStr
+    age  <- Gen.chooseNum(10, 120)
+  } yield Person(name, age)
+
+  val epochDayGen = Gen.chooseNum(LocalDate.MIN.toEpochDay, LocalDate.MAX.toEpochDay)
+
+  val dateGen = epochDayGen.map(LocalDate.ofEpochDay)
+
+  val measurementGen: Gen[Measurement] = for {
+    person <- personGen
+    hr <- hrGen
+    dt <- dateGen
+  } yield Measurement(person, hr, dt)
+
+}
diff --git a/HeartRate/src/test/scala/HeartRateSpec.scala b/HeartRate/src/test/scala/HeartRateSpec.scala
new file mode 100644
index 0000000..c1a4b9b
--- /dev/null
+++ b/HeartRate/src/test/scala/HeartRateSpec.scala
@@ -0,0 +1,38 @@
+package io.whatifs.heartrates
+
+import org.scalatest.{Matchers, PropSpec}
+import org.scalatestplus.scalacheck.ScalaCheckDrivenPropertyChecks
+import eu.timepit.refined.refineV
+import eu.timepit.refined.auto._
+
+class HeartRateSpec
+    extends PropSpec
+    with ScalaCheckDrivenPropertyChecks
+    with Matchers {
+
+  import HeartRateGenerators._
+
+  property ("When building HeartRate, bounds are respected") {
+    forAll(bpmGen) {
+      case bpm if (bpm < HeartRate.MinHeartRate) =>
+        assert(HeartRate.build(bpm).isLeft)
+
+      case bpm if (bpm > HeartRate.MaxHeartRate) =>
+        assert(HeartRate.build(bpm).isLeft)
+
+      case bpm => 
+        val hr = refineV[HeartRate.ValidHeartRate](bpm).map(HeartRate.apply(_))
+        assert(hr.isRight)
+        assertResult(hr)  {
+          HeartRate.build(bpm)
+        }
+    }
+  }
+
+  property ("HeartRate compiles only for legal values") {
+    HeartRate(120)
+    assertCompiles("HeartRate(120)")
+    assertTypeError("HeartRate(-100)")
+  }
+
+}
diff --git a/HeartRate/src/test/scala/MeassurementSpec.scala b/HeartRate/src/test/scala/MeassurementSpec.scala
new file mode 100644
index 0000000..cc1c539
--- /dev/null
+++ b/HeartRate/src/test/scala/MeassurementSpec.scala
@@ -0,0 +1,32 @@
+package io.whatifs.heartrates
+
+import java.time.LocalDate
+
+import org.scalatest.{Matchers, PropSpec}
+import org.scalatestplus.scalacheck.ScalaCheckDrivenPropertyChecks
+
+class MeassurementSpec
+    extends PropSpec
+    with ScalaCheckDrivenPropertyChecks
+    with Matchers {
+
+  import HeartRateGenerators._
+
+  property ("Measurements are done same day") {
+    forAll(personGen, hrGen) { (person, hr) =>
+      val measurement = Measurement.measure(person, hr)
+      assertResult(Measurement(person, hr, LocalDate.now())) {
+        measurement
+      }
+      assert(measurement.warn.isDefined == hr.isHigh, "today measurement warning <==> high heart rate")
+    }
+  }
+
+  property ("Measurement warning properties") {
+    forAll (measurementGen) { measurement =>
+      assertResult(measurement.heartRate.isHigh && measurement.date.isAfter(Measurement.warnIfAfter))
+      { measurement.warn.isDefined }
+    }
+  }
+
+}
\ No newline at end of file
diff --git a/README.md b/README.md
index fff8cc3..c05d164 100644
--- a/README.md
+++ b/README.md
@@ -19,3 +19,5 @@ They are not designed to teach you a technique but to show you an extreme from w
 2. What if you never write a function longer than 3 lines?
 
 3. What if you write non-implementation-specific scenarios in BDD?
+
+4. What if you write all your unit-tests as property based tests?
diff --git a/propertybasedtesting/README.md b/propertybasedtesting/README.md
deleted file mode 100644
index 1398299..0000000
--- a/propertybasedtesting/README.md
+++ /dev/null
@@ -1 +0,0 @@
-Simple Sbt project with Scala.
\ No newline at end of file
diff --git a/propertybasedtesting/project/plugins.sbt b/propertybasedtesting/project/plugins.sbt
deleted file mode 100644
index 9be3b57..0000000
--- a/propertybasedtesting/project/plugins.sbt
+++ /dev/null
@@ -1,7 +0,0 @@
-logLevel := Level.Warn
-
-resolvers += Resolver.sonatypeRepo("releases")
-
-//addSbtPlugin("com.lucidchart" % "sbt-scalafmt" % "1.1")
-
-//addSbtPlugin("de.heikoseeberger" % "sbt-header" % "2.0.0")
diff --git a/propertybasedtesting/src/main/scala/App.scala b/propertybasedtesting/src/main/scala/App.scala
deleted file mode 100644
index e1c210c..0000000
--- a/propertybasedtesting/src/main/scala/App.scala
+++ /dev/null
@@ -1,9 +0,0 @@
-package io.whatifs.propertybasedtesting
-
-case class HeartRate(bpm: Int);
-
-object HeartRate {
-  def build(bpm: Int): Either[String, HeartRate] = {
-    Right(HeartRate(bpm))
-  }
-}
\ No newline at end of file
diff --git a/propertybasedtesting/src/test/scala/AppSpec.scala b/propertybasedtesting/src/test/scala/AppSpec.scala
deleted file mode 100644
index 8c13e71..0000000
--- a/propertybasedtesting/src/test/scala/AppSpec.scala
+++ /dev/null
@@ -1,17 +0,0 @@
-package io.whatifs.propertybasedtesting
-
-import org.scalatest.{Matchers, PropSpec}
-import org.scalatestplus.scalacheck.ScalaCheckDrivenPropertyChecks
-
-class CheckSpec
-  extends PropSpec
-  with ScalaCheckDrivenPropertyChecks
-  with Matchers {
-
-  property ("Heart Rates are always within range") {
-    forAll { (bpm: Int) =>
-      bpm == 1
-    }
-  }
-
-}