diff --git a/README.md b/README.md
index fc48442..01cdc3d 100644
--- a/README.md
+++ b/README.md
@@ -8,4 +8,4 @@ Development occurs in language-specific directories:
 |[Day1.hs](hs/src/Day1.hs)|[Day1.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day1.kt)|[day1.py](py/aoc2024/day1.py)|[day1.rs](rs/src/day1.rs)|
 |[Day2.hs](hs/src/Day2.hs)|[Day2.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day2.kt)|[day2.py](py/aoc2024/day2.py)|[day2.rs](rs/src/day2.rs)|
 |[Day3.hs](hs/src/Day3.hs)|[Day3.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day3.kt)|[day3.py](py/aoc2024/day3.py)|[day3.rs](rs/src/day3.rs)|
-|[Day4.hs](hs/src/Day4.hs)||||
+|[Day4.hs](hs/src/Day4.hs)|[Day4.kt](kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day4.kt)|||
diff --git a/kt/aoc2024-exe/src/commonBench/kotlin/com/github/ephemient/aoc2024/exe/Day4Bench.kt b/kt/aoc2024-exe/src/commonBench/kotlin/com/github/ephemient/aoc2024/exe/Day4Bench.kt
new file mode 100644
index 0000000..4f40e45
--- /dev/null
+++ b/kt/aoc2024-exe/src/commonBench/kotlin/com/github/ephemient/aoc2024/exe/Day4Bench.kt
@@ -0,0 +1,23 @@
+package com.github.ephemient.aoc2024.exe
+
+import com.github.ephemient.aoc2024.Day4
+import kotlinx.benchmark.Benchmark
+import kotlinx.benchmark.Scope
+import kotlinx.benchmark.Setup
+import kotlinx.benchmark.State
+
+@State(Scope.Benchmark)
+class Day4Bench {
+    private lateinit var input: String
+
+    @Setup
+    fun setup() {
+        input = getDayInput(4)
+    }
+
+    @Benchmark
+    fun part1() = Day4(input).part1()
+
+    @Benchmark
+    fun part2() = Day4(input).part2()
+}
diff --git a/kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day4.kt b/kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day4.kt
new file mode 100644
index 0000000..b68f7c1
--- /dev/null
+++ b/kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Day4.kt
@@ -0,0 +1,46 @@
+package com.github.ephemient.aoc2024
+
+class Day4(
+    input: String,
+) {
+    private val lines = input.lines()
+
+    fun part1() = lines.indices.sumOf { y ->
+        lines[y].indices.sumOf { x ->
+            Direction.entries.count { (dx, dy) ->
+                "XMAS".withIndex().all { (i, c) ->
+                    lines.getOrNull(y + i * dy)?.getOrNull(x + i * dx) == c
+                }
+            }
+        }
+    }
+
+    fun part2() = lines.indices.sumOf { y ->
+        lines[y].indices.count { x ->
+            if (lines[y][x] != 'A') return@count false
+            val n = lines.getOrNull(y - 1) ?: return@count false
+            val s = lines.getOrNull(y + 1) ?: return@count false
+            val nw = n.getOrNull(x - 1) ?: return@count false
+            val ne = n.getOrNull(x + 1) ?: return@count false
+            val sw = s.getOrNull(x - 1) ?: return@count false
+            val se = s.getOrNull(x + 1) ?: return@count false
+            (nw == 'M' && se == 'S' || se == 'M' && nw == 'S') &&
+                (sw == 'M' && ne == 'S' || ne == 'M' && sw == 'S')
+        }
+    }
+
+    private enum class Direction(val dx: Int, val dy: Int) {
+        EAST(1, 0),
+        NORTHEAST(1, -1),
+        NORTH(0, -1),
+        NORTHWEST(-1, -1),
+        WEST(-1, 0),
+        SOUTHWEST(-1, 1),
+        SOUTH(0, 1),
+        SOUTHEAST(1, 1),
+        ;
+
+        operator fun component1() = dx
+        operator fun component2() = dy
+    }
+}
diff --git a/kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Days.kt b/kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Days.kt
index a277ec6..1b3b8d8 100644
--- a/kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Days.kt
+++ b/kt/aoc2024-lib/src/commonMain/kotlin/com/github/ephemient/aoc2024/Days.kt
@@ -4,6 +4,7 @@ val days: List<Day> = listOf(
     Day(1, ::Day1, Day1::part1, Day1::part2),
     Day(2, ::Day2, Day2::part1, Day2::part2),
     Day(3, ::Day3, Day3::part1, Day3::part2),
+    Day(4, ::Day4, Day4::part1, Day4::part2),
 )
 
 data class Day(
diff --git a/kt/aoc2024-lib/src/commonTest/kotlin/com/github/ephemient/aoc2024/Day4Test.kt b/kt/aoc2024-lib/src/commonTest/kotlin/com/github/ephemient/aoc2024/Day4Test.kt
new file mode 100644
index 0000000..0a8d440
--- /dev/null
+++ b/kt/aoc2024-lib/src/commonTest/kotlin/com/github/ephemient/aoc2024/Day4Test.kt
@@ -0,0 +1,32 @@
+package com.github.ephemient.aoc2024
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class Day4Test {
+    @Test
+    fun part1() {
+        assertEquals(18, Day4(example).part1())
+    }
+
+    @Test
+    fun part2() {
+        assertEquals(9, Day4(example).part2())
+    }
+
+    companion object {
+        private val example =
+            """
+            |MMMSXXMASM
+            |MSAMXMSMSA
+            |AMXSXMAAMM
+            |MSAMASMSMX
+            |XMASAMXAMM
+            |XXAMMXXAMA
+            |SMSMSASXSS
+            |SAXAMASAAA
+            |MAMMMXMMMM
+            |MXMXAXMASX
+            |""".trimMargin()
+    }
+}