diff --git a/AoC_2025/src/days/day04.rs b/AoC_2025/src/days/day04.rs
new file mode 100644
index 0000000..9d60026
--- /dev/null
+++ b/AoC_2025/src/days/day04.rs
@@ -0,0 +1,66 @@
+use crate::days::Solution;
+use std::cmp::{max, min};
+
+pub struct Day04;
+
+impl Solution for Day04 {
+    type Input = Vec<Vec<bool>>;
+
+    fn parse(&self, data: &str) -> Self::Input {
+        data.split("\n")
+            .filter(|s| !s.is_empty())
+            .map(|s| s.chars().map(|c| c == '@').collect())
+            .collect()
+    }
+
+    fn part1(&self, input: &Self::Input) -> usize {
+        input
+            .iter()
+            .enumerate()
+            .map(|(x, l)| {
+                l.iter()
+                    .enumerate()
+                    .filter(move |(y, v)| {
+                        **v && neighbors((x, *y), input.len())
+                            .iter()
+                            .filter(|(nx, ny)| input[*nx][*ny])
+                            .count()
+                            < 4
+                    })
+                    .count()
+            })
+            .sum()
+    }
+
+    fn part2(&self, input: &Self::Input) -> usize {
+        0
+    }
+}
+
+fn coord_in_range((x, y): (i32, i32), size: usize) -> (usize, usize) {
+    (
+        min(max(x, 0) as usize, size - 1),
+        min(max(y, 0) as usize, size - 1),
+    )
+}
+
+fn neighbors((x, y): (usize, usize), size: usize) -> Vec<(usize, usize)> {
+    (0..=2)
+        .map(|i| i as i32 - 1)
+        .flat_map(|dx| {
+            (0..=2)
+                .map(|i| i as i32 - 1)
+                .filter_map(|dy| {
+                    if dx != 0 || dy != 0 {
+                        Some(coord_in_range((x as i32 + dx, y as i32 + dy), size))
+                    } else {
+                        None
+                    }
+                })
+                .filter(|n| *n != (x, y))
+                .collect::<Vec<(usize, usize)>>()
+        })
+        .collect::<std::collections::HashSet<(usize, usize)>>()
+        .into_iter()
+        .collect::<Vec<(usize, usize)>>()
+}
diff --git a/AoC_2025/src/days/mod.rs b/AoC_2025/src/days/mod.rs
index 3b80959..0d5fd87 100644
--- a/AoC_2025/src/days/mod.rs
+++ b/AoC_2025/src/days/mod.rs
@@ -1,6 +1,7 @@
 pub mod day01;
 pub mod day02;
 pub mod day03;
+pub mod day04;
 
 pub trait Solution {
     type Input;
diff --git a/AoC_2025/src/main.rs b/AoC_2025/src/main.rs
index 01667eb..3c957b5 100644
--- a/AoC_2025/src/main.rs
+++ b/AoC_2025/src/main.rs
@@ -36,6 +36,7 @@ fn main() {
         1 => run(cli.day, days::day01::Day01),
         2 => run(cli.day, days::day02::Day02),
         3 => run(cli.day, days::day03::Day03),
+        4 => run(cli.day, days::day04::Day04),
         _ => {
             eprintln!("Day {:02} is not implemented yet!", cli.day);
             process::exit(1);