diff --git a/src/main/scala/Main.scala b/src/main/scala/Main.scala
index c48535e..f3e079e 100644
--- a/src/main/scala/Main.scala
+++ b/src/main/scala/Main.scala
@@ -5,6 +5,7 @@ import days._
     case 1 => Day1.solve()
     case 2 => Day2.solve()
     case 3 => Day3.solve()
+    case 4 => Day4.solve()
     case _ => println(s"Day $dayNumber is not yet implemented.")
   }
 }
diff --git a/src/main/scala/days/Day4.scala b/src/main/scala/days/Day4.scala
index e066479..7df74c2 100644
--- a/src/main/scala/days/Day4.scala
+++ b/src/main/scala/days/Day4.scala
@@ -1,9 +1,116 @@
 package days
 
+case class Pos(x: Int, y: Int) {
+  def upLeft: Pos = Pos(x - 1, y - 1)
+  def up: Pos = Pos(x, y - 1)
+  def upRight: Pos = Pos(x + 1, y - 1)
+  def left: Pos = Pos(x - 1, y)
+  def right: Pos = Pos(x + 1, y)
+  def downLeft: Pos = Pos(x - 1, y + 1)
+  def down: Pos = Pos(x, y + 1)
+  def downRight: Pos = Pos(x + 1, y + 1)
+
+  def cross: List[Pos] = List(
+    this.upLeft,
+    this.downRight,
+    this.upRight,
+    this.downLeft
+  )
+
+  def inThreshold(geo: Pos): Boolean =
+    x >= 0 && x < geo.x && y >= 0 && y < geo.y
+}
+
+object Pos {
+  def getNeighbors(): List[Pos => Pos] = List(
+    _.upLeft,
+    _.up,
+    _.upRight,
+    _.left,
+    _.right,
+    _.downLeft,
+    _.down,
+    _.downRight
+  )
+}
+
 object Day4 extends Day {
   val number: Int = 4;
 
-  def part1(input: String) = "pouet"
+  def getPositions(p: Pos, geo: Pos, size: Int): List[List[Pos]] =
+    // Get neighbors positions around a position (if matching the geometry)
+    Pos
+      // For each neighbor direction (up-left, up, up-right, etc)
+      .getNeighbors()
+      .map(f =>
+        Iterator
+          // Build an iterator going from [P] to the direction's neighbor [P]
+          .iterate(p)(f)
+          // Do it [size] time
+          .take(size)
+          // Drop positions getting outside the designed dimensions
+          .filter(_.inThreshold(geo))
+          .toList
+      )
+      // Only take the list of positions exactly matching the expected length
+      .filter(_.length == size)
 
-  def part2(input: String) = "pouet"
+  def getCross(p: Pos, geo: Pos): Option[List[Pos]] =
+    // Get surrounding cross around a position (if matching the geometry)
+    p.cross.filter(_.inThreshold(geo)) match {
+      case l if l.length == 4 => Some(l)
+      case _                  => None
+    }
+
+  def getString(lines: List[String])(ps: List[Pos]): String =
+    // Convert list of positions to matching letters
+    ps.map {
+      case Pos(x, y) => {
+        lines(y)(x)
+      }
+    }.mkString
+
+  def isCrossedMAS(s: String): Boolean =
+    // Verify if a string is a crossed MAS
+    s.length == 4
+      && s.forall("MS".contains)
+      && s(0) != s(1)
+      && s(2) != s(3)
+
+  // Verify if a string is a XMAS
+  def isXMAS: String => Boolean = _ == "XMAS"
+
+  def part1(input: String) = {
+    val lines: List[String] = input.linesIterator.toList
+    val geom: Pos = Pos(lines(0).length, lines.length)
+    lines.zipWithIndex
+      .flatMap { case (s, y) =>
+        s.iterator.zipWithIndex.toList
+          .filter(_._1 == 'X')
+          .flatMap { case (_, x) =>
+            getPositions(Pos(x, y), geom, 4)
+              .map(getString(lines))
+              .filter(isXMAS)
+          }
+      }
+      .length
+      .toString
+  }
+
+  def part2(input: String) = {
+    val lines: List[String] = input.linesIterator.toList
+    val geom: Pos = Pos(lines(0).length, lines.length)
+    lines.zipWithIndex
+      .flatMap { case (s, y) =>
+        s.iterator.zipWithIndex.toList
+          .filter(_._1 == 'A')
+          .flatMap { case (_, x) =>
+            getCross(Pos(x, y), geom)
+              .map(getString(lines))
+              .filter(isCrossedMAS)
+          }
+      }
+      .length
+      .toString
+  }
 }