diff --git a/aoc/src/main/scala/aoc/y2019/Day04.scala b/aoc/src/main/scala/aoc/y2019/Day04.scala
new file mode 100644
index 0000000..a03255b
--- /dev/null
+++ b/aoc/src/main/scala/aoc/y2019/Day04.scala
@@ -0,0 +1,40 @@
+package aoc.y2019
+
+import aoc.Day
+
+object Day04 extends Day {
+
+  import fastparse._, SingleLineWhitespace._
+
+  def parseNumber[_: P]: P[Int] = P(CharIn("0-9").rep(1).!).map(_.toInt)
+  def parseRange[_: P]: P[Range.Inclusive] = P(parseNumber ~ "-" ~ parseNumber).map {
+    case (a, b) => a to b
+  }
+
+  def validPassword(password: Int, range: Range, repetitionBounded: Boolean): Boolean = {
+    lazy val isSixDigits = password.toString.length == 6
+    lazy val isWithinRange = range.contains(password)
+    lazy val hasRepetition =
+      if(!repetitionBounded)
+        password.toString.toSeq.groupBy(identity).exists(_._2.length >= 2)
+      else
+        password.toString.toSeq.groupBy(identity).exists(_._2.length == 2)
+    lazy val isIncreasing = password.toString.toSeq.sorted == password.toString.toSeq
+
+    isSixDigits && isWithinRange && hasRepetition && isIncreasing
+  }
+
+  def numberValidPasswords(rangeString: String, repetitionBounded: Boolean): Int = {
+    val range = parse(rangeString, parseRange(_)).get.value
+    range.count(validPassword(_, range, repetitionBounded))
+  }
+
+  override def part1(input: String): String = {
+    numberValidPasswords(input, repetitionBounded = false).toString
+  }
+
+  override def part2(input: String): String = {
+    numberValidPasswords(input, repetitionBounded = true).toString
+  }
+
+}
diff --git a/aoc/src/main/scala/aoc/y2019/package.scala b/aoc/src/main/scala/aoc/y2019/package.scala
index 1bf5377..e677d39 100644
--- a/aoc/src/main/scala/aoc/y2019/package.scala
+++ b/aoc/src/main/scala/aoc/y2019/package.scala
@@ -6,6 +6,7 @@ package object y2019 extends Year {
     "1" -> Day01,
     "2" -> Day02,
     "3" -> Day03,
+    "4" -> Day04,
   )
 
 }