diff --git a/README.md b/README.md
index 52b72a8..0268250 100644
--- a/README.md
+++ b/README.md
@@ -35,3 +35,4 @@ Timings are given as: [lower-bound **best-estimate** upper-bound]
 | 01  | [101.34 µs **101.95 µs** 102.61 µs] | [105.90 µs **106.40 µs** 106.95 µs] |
 | 02  | [2.0990 ms **2.1113 ms** 2.1236 ms] | [2.0954 ms **2.1055 ms** 2.1157 ms] |
 | 03  | [38.717 µs **39.002 µs** 39.311 µs] | [175.35 µs **176.59 µs** 177.92 µs] |
+| 04  | [233.55 µs **234.88 µs** 236.20 µs] | [3.1026 ms **3.1174 ms** 3.1326 ms] |
diff --git a/src/day04/input/test1.txt b/src/day04/input/test1.txt
new file mode 100644
index 0000000..8209399
--- /dev/null
+++ b/src/day04/input/test1.txt
@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
diff --git a/src/day04/mod.rs b/src/day04/mod.rs
new file mode 100644
index 0000000..3d375fc
--- /dev/null
+++ b/src/day04/mod.rs
@@ -0,0 +1,168 @@
+use std::{
+    fmt::{Display, Formatter},
+    str::FromStr,
+};
+
+use color_eyre::{Result, eyre::eyre};
+use tracing::{debug, instrument};
+
+pub const INPUT: &str = include_str!("input/input.txt");
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Cell {
+    Empty,
+    Paper,
+    AccessiblePaper,
+}
+
+impl FromStr for Cell {
+    type Err = color_eyre::Report;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "." => Ok(Cell::Empty),
+            "@" => Ok(Cell::Paper),
+            "x" => Ok(Cell::AccessiblePaper),
+            _ => Err(eyre!("Invalid cell character: {}", s)),
+        }
+    }
+}
+
+impl Cell {
+    fn from_byte(b: u8) -> Result<Self, color_eyre::Report> {
+        match b {
+            b'.' => Ok(Cell::Empty),
+            b'@' => Ok(Cell::Paper),
+            b'x' => Ok(Cell::AccessiblePaper),
+            _ => Err(eyre!("Invalid cell byte: {}", b)),
+        }
+    }
+}
+
+struct Grid<const R: usize, const C: usize> {
+    cells: [[Cell; C]; R],
+}
+
+impl<const R: usize, const C: usize> FromStr for Grid<R, C> {
+    type Err = color_eyre::Report;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        let mut cells = [[Cell::Empty; C]; R];
+
+        for (row, line) in s.lines().enumerate() {
+            for (col, byte) in line.bytes().enumerate() {
+                cells[row][col] = Cell::from_byte(byte)?;
+            }
+        }
+
+        Ok(Grid { cells })
+    }
+}
+
+impl<const R: usize, const C: usize> Display for Grid<R, C> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        for row in 0..R {
+            for col in 0..C {
+                let symbol = match self.cells[row][col] {
+                    Cell::Empty => '.',
+                    Cell::Paper => '@',
+                    Cell::AccessiblePaper => 'x',
+                };
+                write!(f, "{}", symbol)?;
+            }
+            writeln!(f)?;
+        }
+        Ok(())
+    }
+}
+
+impl<const R: usize, const C: usize> Grid<R, C> {
+    fn get_cell(&self, row: isize, col: isize) -> Option<Cell> {
+        if row >= 0 && col >= 0 && (row as usize) < R && (col as usize) < C {
+            Some(self.cells[row as usize][col as usize])
+        } else {
+            None
+        }
+    }
+
+    fn count_accessible_papers(&mut self, replace_with: Cell) -> usize {
+        let mut count = 0;
+        for row in 0..R {
+            for col in 0..C {
+                if self.cells[row][col] != Cell::Paper {
+                    continue;
+                }
+                let mut adjacent_papers = 0;
+                for dr in -1..=1 {
+                    for dc in -1..=1 {
+                        if dr == 0 && dc == 0 {
+                            continue;
+                        }
+                        let adjacent = self
+                            .get_cell(row as isize + dr, col as isize + dc)
+                            .unwrap_or(Cell::Empty);
+                        if adjacent == Cell::Paper || adjacent == Cell::AccessiblePaper {
+                            adjacent_papers += 1;
+                        }
+                    }
+                }
+                if adjacent_papers < 4 {
+                    self.cells[row][col] = replace_with;
+                    count += 1;
+                }
+            }
+        }
+        count
+    }
+}
+
+fn solve_part1<const R: usize, const C: usize>(input: &str) -> Result<usize> {
+    let mut grid = Grid::<R, C>::from_str(input)?;
+    debug!("Parsed grid:\n{}", grid);
+    let count = grid.count_accessible_papers(Cell::AccessiblePaper);
+    debug!("Processed grid:\n{}", grid);
+    Ok(count)
+}
+
+#[instrument(skip(input))]
+pub fn part1(input: &str) -> Result<usize> {
+    solve_part1::<135, 135>(input)
+}
+
+fn solve_part2<const R: usize, const C: usize>(input: &str) -> Result<usize> {
+    let mut grid = Grid::<R, C>::from_str(input)?;
+    debug!("Parsed grid:\n{}", grid);
+    let mut count = 0;
+    loop {
+        let removed = grid.count_accessible_papers(Cell::Empty);
+        if removed == 0 {
+            break;
+        }
+        debug!("Removed {} in grid:\n{}", removed, grid);
+        count += removed;
+    }
+    Ok(count)
+}
+
+#[instrument(skip(input))]
+pub fn part2(input: &str) -> Result<usize> {
+    solve_part2::<135, 135>(input)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test_log::test;
+
+    const TEST_INPUT1: &str = include_str!("input/test1.txt");
+
+    #[test]
+    fn test_part1() {
+        assert_eq!(solve_part1::<10, 10>(TEST_INPUT1).unwrap(), 13);
+    }
+
+    #[test]
+    fn test_part2() {
+        assert_eq!(solve_part2::<10, 10>(TEST_INPUT1).unwrap(), 43);
+    }
+}
diff --git a/src/days.rs b/src/days.rs
index e3734d2..60bf563 100644
--- a/src/days.rs
+++ b/src/days.rs
@@ -8,6 +8,7 @@ macro_rules! all_days {
             1 => day01,
             2 => day02,
             3 => day03,
+            4 => day04,
         }
     };
 }
diff --git a/src/lib.rs b/src/lib.rs
index a000027..d0e2c31 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod day01;
 pub mod day02;
 pub mod day03;
+pub mod day04;
 pub mod days;