diff --git a/.idea/advent_of_code_2022.iml b/.idea/advent_of_code_2022.iml
index a802623..95ca2a2 100644
--- a/.idea/advent_of_code_2022.iml
+++ b/.idea/advent_of_code_2022.iml
@@ -7,6 +7,7 @@
       <sourceFolder url="file://$MODULE_DIR$/day_01/src" isTestSource="false" />
       <sourceFolder url="file://$MODULE_DIR$/day_02/src" isTestSource="false" />
       <sourceFolder url="file://$MODULE_DIR$/day_03/src" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/day_04/src" isTestSource="false" />
       <excludeFolder url="file://$MODULE_DIR$/target" />
     </content>
     <orderEntry type="inheritedJdk" />
diff --git a/Cargo.toml b/Cargo.toml
index 9246031..2ebb35e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,2 +1,2 @@
 [workspace]
-members = ['common', 'cli', 'day_01', 'day_02', 'day_03']
+members = ['common', 'cli', 'day_01', 'day_02', 'day_03', 'day_04']
diff --git a/cli/Cargo.toml b/cli/Cargo.toml
index 5736a0c..b0cf484 100644
--- a/cli/Cargo.toml
+++ b/cli/Cargo.toml
@@ -10,4 +10,5 @@ clap = { version = "4.0", features = ["derive"] }
 day_01 = { path = "../day_01"}
 day_02 = { path = "../day_02"}
 day_03 = { path = "../day_03"}
+day_04 = { path = "../day_04"}
 common = { path = "../common"}
diff --git a/cli/src/main.rs b/cli/src/main.rs
index f98db12..ea9c1ae 100644
--- a/cli/src/main.rs
+++ b/cli/src/main.rs
@@ -71,6 +71,21 @@ fn main() -> Result<(), Box<dyn Error>> {
             );
             Ok(())
         }
+        (day @ Day::Four, part @ Part::One) => {
+            println!(
+                "{}",
+                day_04::run_day_and_part(day, part, input_str.as_str())
+            );
+            Ok(())
+        }
+        (day @ Day::Four, part @ Part::Two) => {
+            println!(
+                "{}",
+                day_04::run_day_and_part(day, part, input_str.as_str())
+            );
+            Ok(())
+        }
+
 
         (_, _) => {
             unimplemented!()
diff --git a/common/src/lib.rs b/common/src/lib.rs
index 72d951c..1ee9994 100644
--- a/common/src/lib.rs
+++ b/common/src/lib.rs
@@ -5,6 +5,7 @@ pub enum Day {
     One,
     Two,
     Three,
+    Four,
 }
 
 #[derive(ValueEnum, Copy, Clone, Debug)]
diff --git a/day_04/Cargo.toml b/day_04/Cargo.toml
new file mode 100644
index 0000000..1c6fabb
--- /dev/null
+++ b/day_04/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "day_04"
+version = "0.0.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+common = {path = '../common'}
+nom = "7.1.1"
diff --git a/day_04/input/input b/day_04/input/input
new file mode 100644
index 0000000..f8b658d
--- /dev/null
+++ b/day_04/input/input
@@ -0,0 +1,1000 @@
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diff --git a/day_04/src/lib.rs b/day_04/src/lib.rs
new file mode 100644
index 0000000..b29b21c
--- /dev/null
+++ b/day_04/src/lib.rs
@@ -0,0 +1,225 @@
+use common::{Day, Part};
+use nom::character::complete::{char, digit1, line_ending};
+use nom::multi::separated_list1;
+use nom::sequence::separated_pair;
+use nom::IResult;
+use std::ops::RangeInclusive;
+
+pub fn run_day_and_part(day: Day, part: Part, inp: &str) -> usize {
+    match (day, part) {
+        (Day::Four, Part::One) => {
+            part_01_do(inp)
+                .into_iter()
+                .fold(0, |acc, it| if it { acc + 1 } else { acc })
+        }
+        (Day::Four, Part::Two) => {
+            part_02_do(inp)
+                .into_iter()
+                .fold(0, |acc, it| if it { acc + 1 } else { acc })
+        }
+        (_, _) => unimplemented!(),
+    }
+}
+
+fn elf_range(s: &str) -> IResult<&str, RangeInclusive<usize>> {
+    let (s, elf_range): (&str, (&str, &str)) = separated_pair(digit1, char('-'), digit1)(s)?;
+
+    let left: usize = elf_range.0.parse::<_>().unwrap();
+
+    let right: usize = elf_range.1.parse::<_>().unwrap();
+    Ok((s, RangeInclusive::new(left, right)))
+}
+
+#[derive(Debug, Eq, PartialEq, Clone)]
+struct ElfRangeLine {
+    first: RangeInclusive<usize>,
+    second: RangeInclusive<usize>,
+}
+
+impl ElfRangeLine {
+    fn one_contained_in_other(&self) -> bool {
+        self.first.clone().all(|a| self.second.clone().contains(&a))
+            || self.second.clone().all(|a| self.first.clone().contains(&a))
+    }
+    fn any_overlap(&self) -> bool {
+        self.first.clone().any(|a| self.second.clone().contains(&a))
+            || self.second.clone().any(|a| self.first.clone().contains(&a))
+    }
+}
+
+fn elf_range_line(s: &str) -> IResult<&str, ElfRangeLine> {
+    let (s, elf_ranges): (&str, (RangeInclusive<usize>, RangeInclusive<usize>)) =
+        separated_pair(elf_range, char(','), elf_range)(s)?;
+
+    Ok((
+        s,
+        ElfRangeLine {
+            first: elf_ranges.0,
+            second: elf_ranges.1,
+        },
+    ))
+}
+
+fn elf_range_sequence(s: &str) -> IResult<&str, Vec<ElfRangeLine>> {
+    separated_list1(line_ending, elf_range_line)(s)
+}
+
+fn part_01_do(inp: &str) -> Vec<bool> {
+    let (_, ranges): (_, Vec<ElfRangeLine>) = elf_range_sequence(inp).unwrap();
+
+    ranges
+        .iter()
+        .map(|r: &ElfRangeLine| r.one_contained_in_other())
+        .collect()
+}
+
+fn part_02_do(inp: &str) -> Vec<bool> {
+    let (_, ranges): (_, Vec<ElfRangeLine>) = elf_range_sequence(inp).unwrap();
+
+    ranges
+        .iter()
+        .map(|r: &ElfRangeLine| r.any_overlap())
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn elf_range_creation() {
+        assert_eq!(Ok(("", (0..=4))), elf_range("0-4"))
+    }
+
+    #[test]
+    fn elf_range_line_creation() {
+        assert_eq!(
+            Ok((
+                "",
+                ElfRangeLine {
+                    first: (0..=4),
+                    second: (2..=5)
+                }
+            )),
+            elf_range_line("0-4,2-5")
+        )
+    }
+
+    #[test]
+    fn elf_range_contains_works_in_obvious_case() {
+        let expected = true;
+
+        let under_test = ElfRangeLine {
+            first: (2..=8),
+            second: (3..=7),
+        };
+        let res = under_test.one_contained_in_other();
+
+        assert_eq!(expected, res)
+    }
+
+    #[test]
+    fn elf_range_contains_fails_correctly() {
+        let expected = false;
+
+        let under_test = ElfRangeLine {
+            first: (2..=8),
+            second: (3..=9),
+        };
+        let res = under_test.one_contained_in_other();
+
+        assert_eq!(expected, res)
+    }
+
+    #[test]
+    fn elf_range_contains_reflexive() {
+        let expected = true;
+
+        let under_test = ElfRangeLine {
+            first: (3..=7),
+            second: (2..=8),
+        };
+        let res = under_test.one_contained_in_other();
+
+        assert_eq!(expected, res)
+    }
+
+    #[test]
+    fn part_01_example_parsed_correctly() {
+        let expected = Ok((
+            "",
+            vec![
+                ElfRangeLine {
+                    first: (2..=4),
+                    second: (6..=8),
+                },
+                ElfRangeLine {
+                    first: (2..=3),
+                    second: (4..=5),
+                },
+                ElfRangeLine {
+                    first: (5..=7),
+                    second: (7..=9),
+                },
+                ElfRangeLine {
+                    first: (2..=8),
+                    second: (3..=7),
+                },
+                ElfRangeLine {
+                    first: (6..=6),
+                    second: (4..=6),
+                },
+                ElfRangeLine {
+                    first: (2..=6),
+                    second: (4..=8),
+                },
+            ],
+        ));
+
+        assert_eq!(
+            expected,
+            elf_range_sequence(
+                r#"2-4,6-8
+2-3,4-5
+5-7,7-9
+2-8,3-7
+6-6,4-6
+2-6,4-8"#
+            )
+        )
+    }
+
+    #[test]
+    fn part_01_example_score_is_correct() {
+        assert_eq!(
+            2,
+            run_day_and_part(
+                Day::Four,
+                Part::One,
+                r#"2-4,6-8
+2-3,4-5
+5-7,7-9
+2-8,3-7
+6-6,4-6
+2-6,4-8"#
+            )
+        )
+    }
+
+    #[test]
+    fn part_02_example_score_is_correct() {
+        assert_eq!(
+            4,
+            run_day_and_part(
+                Day::Four,
+                Part::Two,
+                r#"2-4,6-8
+2-3,4-5
+5-7,7-9
+2-8,3-7
+6-6,4-6
+2-6,4-8"#
+            )
+        )
+    }
+}