diff --git a/app/src/main/kotlin/day03/Day03.kt b/app/src/main/kotlin/day03/Day03.kt
index c073bdd..f5cda17 100644
--- a/app/src/main/kotlin/day03/Day03.kt
+++ b/app/src/main/kotlin/day03/Day03.kt
@@ -1,13 +1,8 @@
-
 package day03
 
 import kotlin.io.println
 
 class Day03 {
-    fun parse(input: String): String {
-        throw NotImplementedError()
-    }
-
     fun part1(input: String): Int {
         val regex = Regex("mul\\((\\d{1,3}),(\\d{1,3})\\)")
         val muls = regex.findAll(input)
@@ -47,6 +42,5 @@ class Day03 {
         .sum()
 
         return product
-
     }
 }
diff --git a/app/src/main/kotlin/day04/Day04.kt b/app/src/main/kotlin/day04/Day04.kt
new file mode 100644
index 0000000..40077f5
--- /dev/null
+++ b/app/src/main/kotlin/day04/Day04.kt
@@ -0,0 +1,68 @@
+package day04
+
+import kotlin.io.println
+
+class Day04 {
+    fun part1(input: String): Int {
+        val lines = input.lines().map { it.toList() }
+        var count = 0
+        for (x in 0..<lines.count()) {
+            for (y in 0..<lines[x].count()) {
+                if (lines[x][y] == 'X') {
+                    count += checkMASPart1(x, y, lines)
+                }
+            }
+        }
+        return count
+    }
+
+    fun checkMASPart1(x: Int, y: Int, lines: List<List<Char>>): Int {
+        val maxX = lines.count() - 1
+        val maxY = lines[0].count() - 1
+        var count = 0
+
+        for (dX in -1..1) {
+            if (x+(dX*3) < 0 || x+(dX*3) > maxX) { continue }
+            for (dY in -1..1) {
+                if (y+(dY*3) < 0 || y+(dY*3) > maxY) { continue }
+                if (lines[x+dX][y+dY] == 'M' && lines[x+(dX*2)][y+(dY*2)] == 'A' && lines[x+(dX*3)][y+(dY*3)] == 'S') {
+                    count += 1
+                }
+            }
+        }
+        return count
+    }
+
+    fun part2(input: String): Int {
+        val lines = input.lines().map { it.toList() }
+        var count = 0
+        for (x in 0..<lines.count()) {
+            for (y in 0..<lines[x].count()) {
+                if (lines[x][y] == 'A') {
+                    count += checkMASPart2(x, y, lines)
+                }
+            }
+        }
+        return count
+    }
+
+    fun checkMASPart2(x: Int, y: Int, lines: List<List<Char>>): Int {
+        val maxX = lines.count() - 1
+        val maxY = lines[0].count() - 1
+        if (x-1 < 0 || x+1 > maxX) { return 0 }
+        if (y-1 < 0 || y+1 > maxY) { return 0 }
+        val topLeft = lines[x-1][y-1]
+        val bottomRight = lines[x+1][y+1]
+        if (topLeft != bottomRight && (bottomRight == 'M' || bottomRight == 'S') && (topLeft == 'M' || topLeft == 'S')) {
+            val topRight = lines[x-1][y+1]
+            val bottomLeft = lines[x+1][y-1]
+            if (topRight != bottomLeft && (bottomLeft == 'M' || bottomLeft == 'S') && (topRight == 'M' || topRight == 'S')) {
+                return 1
+            }
+        }
+
+        return 0
+    }
+
+}
+
diff --git a/app/src/test/kotlin/day03/Day03Test.kt b/app/src/test/kotlin/day03/Day03Test.kt
index 5b37406..81eb4b7 100644
--- a/app/src/test/kotlin/day03/Day03Test.kt
+++ b/app/src/test/kotlin/day03/Day03Test.kt
@@ -1,4 +1,3 @@
-
 package day03
 
 import kotlin.test.Test
diff --git a/app/src/test/kotlin/day04/Day04Test.kt b/app/src/test/kotlin/day04/Day04Test.kt
new file mode 100644
index 0000000..4c22334
--- /dev/null
+++ b/app/src/test/kotlin/day04/Day04Test.kt
@@ -0,0 +1,49 @@
+package day04
+
+import kotlin.test.Test
+import util.InputDownloader
+
+class Day04Test {
+    val dayNum = 4
+    val day = Day04()
+
+    val input = InputDownloader().getInput(dayNum)
+    val example = """MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX"""
+
+    @Test fun part1Example() {
+        part1(example, 18)
+    }
+
+    @Test fun part1Solution() {
+        part1(input, 2545)
+    }
+
+    @Test fun part2Example() {
+        part2(example, 9)
+    }
+
+    @Test fun part2Solution() {
+        part2(input, 1886)
+    }
+
+    fun part1(input: String, expected: Int) {
+        val output = day.part1(input)
+        println("output: $output")
+        assert(output == expected)
+    }
+
+    fun part2(input: String, expected: Int) {
+        val output = day.part2(input)
+        println("output: $output")
+        assert(output == expected)
+    }
+}