diff --git a/README.md b/README.md
index b6548cc..08a2510 100644
--- a/README.md
+++ b/README.md
@@ -19,11 +19,12 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 
 | Day | Part 1 | Part 2 |
 | :---: | :---: | :---:  |
-| [Day 1](./src/bin/01.rs) | `36.9µs` | `82.8µs` |
-| [Day 2](./src/bin/02.rs) | `44.4µs` | `43.1µs` |
-| [Day 3](./src/bin/03.rs) | `116.0µs` | `113.7µs` |
+| [Day 1](./src/bin/01.rs) | `38.2µs` | `81.2µs` |
+| [Day 2](./src/bin/02.rs) | `43.7µs` | `43.2µs` |
+| [Day 3](./src/bin/03.rs) | `115.3µs` | `113.3µs` |
+| [Day 4](./src/bin/04.rs) | `52.3µs` | `55.3µs` |
 
-**Total: 0.44ms**
+**Total: 0.54ms**
 <!--- benchmarking table --->
 
 ---
diff --git a/data/examples/04.txt b/data/examples/04.txt
new file mode 100644
index 0000000..71f208a
--- /dev/null
+++ b/data/examples/04.txt
@@ -0,0 +1,6 @@
+Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
\ No newline at end of file
diff --git a/src/bin/04.rs b/src/bin/04.rs
new file mode 100644
index 0000000..cb7dd89
--- /dev/null
+++ b/src/bin/04.rs
@@ -0,0 +1,63 @@
+advent_of_code::solution!(4);
+
+pub fn part_one(input: &str) -> Option<u32> {
+    Some(
+        input
+            .lines()
+            .filter_map(winning_number_count)
+            .filter(|&c| c > 0)
+            .map(|c| 1 << (c - 1))
+            .sum(),
+    )
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let lines = input.lines().collect::<Vec<_>>();
+    let mut copies = vec![1; lines.len()];
+    let mut total = 0;
+    for (index, line) in lines.into_iter().enumerate() {
+        let card_copies = copies[index];
+        total += card_copies;
+        let winning_numbers = winning_number_count(line).unwrap_or(0) as usize;
+        for i in 0..winning_numbers {
+            if let Some(elem) = copies.get_mut(index + i + 1) {
+                *elem += card_copies;
+            }
+        }
+    }
+    Some(total)
+}
+
+fn winning_number_count(card: &str) -> Option<u32> {
+    let (_, numbers_part) = card.split_once(':')?;
+    let (winning_str, have_str) = numbers_part.split_once('|')?;
+    let mut winning_numbers = [false; 100];
+    for s in winning_str.split_whitespace() {
+        winning_numbers[s.parse::<usize>().ok()?] = true;
+    }
+    let mut count = 0;
+    for s in have_str.split_whitespace() {
+        let num = s.parse::<usize>().ok()?;
+        if winning_numbers[num] {
+            count += 1;
+        }
+    }
+    Some(count)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(13));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(30));
+    }
+}