diff --git a/data/examples/04.txt b/data/examples/04.txt
new file mode 100644
index 0000000..0355ad3
--- /dev/null
+++ b/data/examples/04.txt
@@ -0,0 +1,5 @@
+aaaaa-bbb-z-y-x-123[abxyz]
+a-b-c-d-e-f-g-h-987[abcde]
+not-a-real-room-404[oarel]
+totally-real-room-200[decoy]
+qzmt-zixmtkozy-ivhz-343[zimth]
diff --git a/src/bin/04.rs b/src/bin/04.rs
new file mode 100644
index 0000000..b9848df
--- /dev/null
+++ b/src/bin/04.rs
@@ -0,0 +1,122 @@
+advent_of_code::solution!(4);
+
+use regex::Regex;
+use std::cmp::Ordering;
+use std::collections::HashMap;
+
+struct Entry {
+    room: u32,
+    checksum: Vec<char>,
+    name: Vec<char>,
+}
+
+fn rotate_char(c: char, shift: u8) -> char {
+    let a: u8 = b'a';
+
+    match c {
+        '-' => ' ',
+        'a'..='z' => (((c as u8) - a + shift) % 26 + a) as char,
+        _ => c,
+    }
+}
+
+impl Entry {
+    fn from(s: &str) -> Entry {
+        let re = Regex::new(r"^(([a-z]+-)+)(\d+)\[([a-z]{5})\]$").unwrap();
+        if let Some(capture) = re.captures(s) {
+            let (_, [name, _, room, checksum]) = capture.extract();
+            if checksum.len() != 5 {
+                println!("input: {:?}, parsed checksum: {:?}", s, checksum);
+                panic!("invalid checksum length");
+            }
+            return Entry {
+                room: room.parse::<u32>().unwrap(),
+                checksum: checksum.chars().collect(),
+                name: name.chars().collect(),
+            };
+        }
+        println!("{:?}", s);
+        panic!("invalid input");
+    }
+
+    fn count_letters(&self) -> Vec<(char, usize)> {
+        let mut counts: HashMap<char, usize> = HashMap::new();
+        for c in &self.name {
+            *counts.entry(*c).or_default() += 1;
+        }
+        counts.into_iter().filter(|(c, _)| *c != '-').collect()
+    }
+
+    fn is_valid(&self) -> bool {
+        let mut letters_count = self.count_letters();
+        letters_count.sort_by(|(chr1, cnt1), (chr2, cnt2)| match cnt2.cmp(cnt1) {
+            Ordering::Equal => chr1.cmp(chr2),
+            cmp => cmp,
+        });
+        let letters_count: Vec<char> = letters_count
+            .into_iter()
+            .take(5)
+            .map(|(chr, _)| chr)
+            .collect();
+
+        letters_count == self.checksum
+    }
+
+    fn rotated_name(&self) -> String {
+        let shift: u8 = (self.room % 26) as u8;
+        (*self.name)
+            .iter()
+            .map(|c| rotate_char(*c, shift))
+            .collect()
+    }
+}
+
+fn parse_input(input: &str) -> Vec<Entry> {
+    input
+        .strip_suffix("\n")
+        .unwrap_or(input)
+        .split("\n")
+        .map(Entry::from)
+        .collect()
+}
+
+pub fn part_one(input: &str) -> Option<u32> {
+    Some(
+        parse_input(input)
+            .iter()
+            .filter(|e| e.is_valid())
+            .map(|e| e.room)
+            .sum(),
+    )
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    let res: Vec<_> = parse_input(input)
+        .iter()
+        .filter(|e| e.is_valid())
+        .map(|e| (e.rotated_name(), e.room))
+        .collect();
+    for (name, room) in res {
+        if name.as_str().starts_with("north") {
+            return Some(room);
+        }
+    }
+    None
+}
+
+#[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(1857));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, None);
+    }
+}