diff --git a/lib/common.ex b/lib/common.ex
index 3c6419b..899fbbb 100644
--- a/lib/common.ex
+++ b/lib/common.ex
@@ -32,4 +32,6 @@ defmodule Aoc2023.Common do
   end
 
   def id(x), do: x
+  def const(_, x), do: x
+  def lines(x), do: String.split(x, "\n")
 end
diff --git a/lib/day4.ex b/lib/day4.ex
new file mode 100644
index 0000000..16e7160
--- /dev/null
+++ b/lib/day4.ex
@@ -0,0 +1,39 @@
+defmodule Aoc2023.Day4 do
+  use Aoc2023
+
+  defp numbers(values) do
+    values |> String.split() |> Enum.map(&String.to_integer/1) |> MapSet.new()
+  end
+
+  def parse(input) do
+    input
+    |> lines
+    |> Enum.map(
+      &Regex.run(~r/Card\s+(?<id>\d+): (?<winning>(\s*\d+)+) \| (?<values>(\s*\d+)+)/, &1,
+        capture: ["id", "winning", "values"]
+      )
+    )
+    |> Enum.map(fn [id, winning, values] ->
+      {String.to_integer(id),
+       MapSet.intersection(numbers(winning), numbers(values)) |> MapSet.size()}
+    end)
+  end
+
+  def part1(input) do
+    for {_, count} <- input,
+        count > 0 do
+      2 ** (count - 1)
+    end
+    |> Enum.sum()
+  end
+
+  def part2(input) do
+    for {id, count} <- input,
+        id_ <- (id + 1)..(id + count)//1,
+        reduce: input |> Enum.map(elem(0) &&& const(1)) |> Enum.into(%{}) do
+      acc -> Map.update!(acc, id_, &(&1 + Map.get(acc, id)))
+    end
+    |> Enum.map(pipe(elem(1)))
+    |> Enum.sum()
+  end
+end
diff --git a/lib/mix_tasks.ex b/lib/mix_tasks.ex
index 2482d83..59cb84e 100644
--- a/lib/mix_tasks.ex
+++ b/lib/mix_tasks.ex
@@ -5,6 +5,7 @@ defmodule Mix.Tasks.Aoc do
   defp module(1), do: Aoc2023.Day1
   defp module(2), do: Aoc2023.Day2
   defp module(3), do: Aoc2023.Day3
+  defp module(4), do: Aoc2023.Day4
   # [MODULE INSERTION POINT]  
 
   defp base_dir(), do: System.get_env("AOC_BASE")
@@ -102,7 +103,7 @@ defmodule Mix.Tasks.Aoc do
   end
 
   defp run(day, ["test", "watch"]) do
-    {:ok, pid} = Mix.Tasks.Aoc.Watcher.start_link({[dirs: ["lib/", "test/day#{day}/"]], self()})
+    {:ok, pid} = Mix.Tasks.Aoc.Watcher.start_link({[dirs: ["lib/", "tests/day#{day}/"]], self()})
     Process.monitor(pid)
     loop = fn (loop) ->
       IO.puts(IO.ANSI.clear() <> IO.ANSI.cursor(0, 0))
@@ -112,7 +113,9 @@ defmodule Mix.Tasks.Aoc do
       :code.purge(Aoc2023.Common)
       :code.load_file(module(day))
       :code.load_file(Aoc2023.Common)
-      run(day, ["test"])
+      spawn fn ->
+        run(day, ["test"])
+      end
       receive do
         :update -> loop.(loop)
         {:DOWN, _, :process, ^pid, _} -> nil
@@ -129,8 +132,6 @@ defmodule Mix.Tasks.Aoc do
     |> Enum.map(&test(day, &1))
     |> Enum.map(fn {test, input, p1e, p2e} ->
       parsed = mod.parse(input)
-      p1 = mod.part1(parsed)
-      p2 = mod.part2(parsed)
 
       result = fn (part, got, expected) ->
         cond do
@@ -142,8 +143,8 @@ defmodule Mix.Tasks.Aoc do
         end
       end
 
-      result.(1, p1, p1e)
-      result.(2, p2, p2e)
+      result.(1, mod.part1(parsed), p1e)
+      result.(2, mod.part2(parsed), p2e)
     end)
   end
 
diff --git a/tests/day4/1 b/tests/day4/1
new file mode 100644
index 0000000..9bdb874
--- /dev/null
+++ b/tests/day4/1
@@ -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
diff --git a/tests/day4/1.1 b/tests/day4/1.1
new file mode 100644
index 0000000..b1bd38b
--- /dev/null
+++ b/tests/day4/1.1
@@ -0,0 +1 @@
+13
diff --git a/tests/day4/1.2 b/tests/day4/1.2
new file mode 100644
index 0000000..64bb6b7
--- /dev/null
+++ b/tests/day4/1.2
@@ -0,0 +1 @@
+30