diff --git a/2021/day4.exs b/2021/day4.exs
new file mode 100644
index 0000000..457444e
--- /dev/null
+++ b/2021/day4.exs
@@ -0,0 +1,119 @@
+defmodule Day4 do
+  @rows for y <- 0..4, do: for(x <- 0..4, do: {x, y})
+  @cols for x <- 0..4, do: for(y <- 0..4, do: {x, y})
+
+  def part1(input) do
+    {current_number, winning_board} = play(input)
+    sum_unmarked(winning_board) * current_number
+  end
+
+  def play({_numbers = [], boards}), do: boards
+
+  def play({[current_number | next_numbers], boards}) do
+    boards = for board <- boards, do: mark(board, current_number)
+
+    case check_for_winner(boards) do
+      nil -> play({next_numbers, boards})
+      board -> {current_number, board}
+    end
+  end
+
+  def sum_unmarked(board) do
+    board
+    |> Enum.filter(&match?({_key, {_number, false}}, &1))
+    |> Enum.map(fn {_key, {number, _marked}} -> number end)
+    |> Enum.sum()
+  end
+
+  def mark(board, current_number) do
+    case Enum.find(board, &match?({_key, {^current_number, false}}, &1)) do
+      nil -> board
+      {key, _value} -> Map.put(board, key, {current_number, true})
+    end
+  end
+
+  def check_for_winner([]), do: nil
+
+  def check_for_winner([board | boards]) do
+    row_complete? =
+      Enum.any?(@rows, fn row ->
+        values = for key <- row, do: board[key]
+        Enum.all?(values, &match?({_number, true}, &1))
+      end)
+
+    col_complete? =
+      Enum.any?(@cols, fn col ->
+        values = for key <- col, do: board[key]
+        Enum.all?(values, &match?({_number, true}, &1))
+      end)
+
+    if row_complete? or col_complete? do
+      board
+    else
+      check_for_winner(boards)
+    end
+  end
+
+  def part2(_input) do
+    :ok
+  end
+
+  def input do
+    with [input_filename] <- System.argv(),
+         {:ok, input} <- File.read(input_filename) do
+      [numbers | boards] = String.split(input, "\n", trim: true)
+      numbers = numbers |> String.split(",") |> Enum.map(&String.to_integer/1)
+      boards = parse_boards(boards)
+      {numbers, boards}
+    else
+      _ -> :error
+    end
+  end
+
+  defp parse_boards([]), do: []
+
+  defp parse_boards(boards) do
+    {board, boards} = Enum.split(boards, 5)
+
+    board = Enum.map(board, &String.split/1)
+
+    board =
+      for {row, y} <- Enum.with_index(board), {value, x} <- Enum.with_index(row), into: %{} do
+        {{x, y}, {String.to_integer(value), false}}
+      end
+
+    [board | parse_boards(boards)]
+  end
+
+  #######################
+  # HERE BE BOILERPLATE #
+  #######################
+
+  def run do
+    case input() do
+      :error -> print_usage()
+      input -> run_parts_with_timer(input)
+    end
+  end
+
+  defp run_parts_with_timer(input) do
+    run_with_timer(1, fn -> part1(input) end)
+    run_with_timer(2, fn -> part2(input) end)
+  end
+
+  defp run_with_timer(part, fun) do
+    {time, result} = :timer.tc(fun)
+    IO.puts("Part #{part} (completed in #{format_time(time)}):\n")
+    IO.puts("#{result}\n")
+  end
+
+  defp format_time(μsec) when μsec < 1_000, do: "#{μsec}μs"
+  defp format_time(μsec) when μsec < 1_000_000, do: "#{μsec / 1000}ms"
+  defp format_time(μsec), do: "#{μsec / 1_000_000}s"
+
+  defp print_usage do
+    IO.puts("Usage: elixir day4.exs input_filename")
+  end
+end
+
+# Day4.run()