diff --git a/utils/src/grid.rs b/utils/src/grid.rs
--- a/utils/src/grid.rs
+++ b/utils/src/grid.rs
@@ -156,6 +156,13 @@ .get(pos.y as usize)
             .and_then(|row| row.get(pos.x as usize))
     }
 
+    /// Obtain a mutable reference to a tile in the grid
+    pub fn get_mut<'a>(&'a mut self, pos: Position) -> Option<&'a mut T> {
+        self.data
+            .get_mut(pos.y as usize)
+            .and_then(|row| row.get_mut(pos.x as usize))
+    }
+
     /// Get a value from the grid at the given position,
     /// panicking if the position is out of bounds.
     ///
diff --git a/years/2025/src/day_2.rs b/years/2025/src/day_2.rs
--- a/years/2025/src/day_2.rs
+++ b/years/2025/src/day_2.rs
@@ -28,7 +28,7 @@ .into_iter()
             .flat_map(|r| {
                 r.into_iter().filter(|x| {
                     let digs = num_digits(*x);
-                    (2..=digs).into_iter().filter(|n| digs % n == 0).any(|n| {
+                    (2..=digs).filter(|n| digs.is_multiple_of(*n)).any(|n| {
                         let mut parts = Vec::with_capacity(n);
                         let mut current = *x;
                         let split_at = digs / n;
@@ -40,7 +40,7 @@ }
                         parts
                             .first()
                             .copied()
-                            .map_or(false, |first| parts.into_iter().all(|x| x == first))
+                            .is_some_and(|first| parts.into_iter().all(|x| x == first))
                     })
                 })
             })
diff --git a/years/2025/src/day_4.rs b/years/2025/src/day_4.rs
--- a/years/2025/src/day_4.rs
+++ b/years/2025/src/day_4.rs
@@ -1,17 +1,65 @@
-
 use advent_core::{Day, day_stuff, ex_for_day};
+use utils::{dir::ALL_8, tiles};
 
 pub struct Day4;
 
+tiles!(Tile, [
+  '@' => Paper,
+  '.' => Empty
+]);
+
+type Grid = utils::grid::Grid<Tile>;
+
 impl Day for Day4 {
+    day_stuff!(4, "13", "43", Grid);
 
-    day_stuff!(4, "", "");
+    fn part_1(input: Self::Input) -> Option<String> {
+        let ans = input
+            .iter()
+            .filter(|(_, t)| **t == Tile::Paper)
+            .filter(|(pos, _)| {
+                input
+                    .relatives(*pos, &ALL_8)
+                    .filter(|(_, _, t)| **t == Tile::Paper)
+                    .count()
+                    < 4
+            })
+            .count();
 
-    fn part_1(_input: Self::Input) -> Option<String> {
-        None
+        Some(ans.to_string())
     }
 
-    fn part_2(_input: Self::Input) -> Option<String> {
-        None
+    fn part_2(mut input: Self::Input) -> Option<String> {
+        let mut i = 0;
+
+        loop {
+            let next = input
+                .iter()
+                .filter(|(_, t)| **t == Tile::Paper)
+                .filter(|(pos, _)| {
+                    input
+                        .relatives(*pos, &ALL_8)
+                        .filter(|(_, _, t)| **t == Tile::Paper)
+                        .count()
+                        < 4
+                })
+                .map(|(p, _)| p)
+                .collect::<Vec<_>>();
+
+            if next.is_empty() {
+                break;
+            } else {
+                i += next.len();
+                for pos in next {
+                    *(input.get_mut(pos).unwrap()) = Tile::Empty;
+                }
+            }
+        }
+
+        Some(i.to_string())
+    }
+
+    fn parse_input(input: &str) -> Self::Input {
+        Grid::parse(input)
     }
 }
diff --git a/years/2025/src/examples/day_4/1.txt b/years/2025/src/examples/day_4/1.txt
--- a/years/2025/src/examples/day_4/1.txt
+++ b/years/2025/src/examples/day_4/1.txt
@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
diff --git a/years/2025/src/examples/day_4/2.txt b/years/2025/src/examples/day_4/2.txt
--- a/years/2025/src/examples/day_4/2.txt
+++ b/years/2025/src/examples/day_4/2.txt
@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.