diff --git a/2021/aoc/aoc.go b/2021/aoc/aoc.go
index 78b46b6..470503b 100644
--- a/2021/aoc/aoc.go
+++ b/2021/aoc/aoc.go
@@ -3,6 +3,8 @@ package aoc
 import (
 	"bufio"
 	"os"
+	"strconv"
+	"strings"
 )
 
 func ReadLines(path string) (lines []string) {
@@ -18,3 +20,19 @@ func ReadLines(path string) (lines []string) {
 	}
 	return lines
 }
+
+func ReadFile(path string) string {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		panic(err)
+	}
+	return strings.TrimSpace(string(b))
+}
+
+func MustInt(s string) int {
+	i, err := strconv.Atoi(s)
+	if err != nil {
+		panic(err)
+	}
+	return i
+}
diff --git a/2021/day4/main.go b/2021/day4/main.go
new file mode 100644
index 0000000..5d2f778
--- /dev/null
+++ b/2021/day4/main.go
@@ -0,0 +1,148 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/fatih/color"
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+type input struct {
+	numbers []int
+	boards  []board
+}
+
+func parseInput(text string) input {
+	sections := strings.Split(text, "\n\n")
+	var nums []int
+	for _, n := range strings.Split(sections[0], ",") {
+		nums = append(nums, aoc.MustInt(n))
+	}
+	var boards []board
+	for _, block := range sections[1:] {
+		b := newBoard()
+		for r, row := range strings.Split(block, "\n") {
+			for c, n := range strings.Fields(row) {
+				b.nums[r][c] = aoc.MustInt(n)
+			}
+		}
+		boards = append(boards, b)
+	}
+	return input{numbers: nums, boards: boards}
+}
+
+type board struct {
+	nums   *[5][5]int
+	marked *[5][5]bool
+	won    bool
+}
+
+func newBoard() board {
+	return board{
+		nums:   &[5][5]int{},
+		marked: &[5][5]bool{},
+	}
+}
+
+func (b board) String() string {
+	var s strings.Builder
+	for r, row := range b.nums {
+		for c, n := range row {
+			if b.marked[r][c] {
+				color.New(color.FgGreen, color.Bold).Fprintf(&s, "%2d ", n)
+			} else {
+				fmt.Fprintf(&s, "%2d ", n)
+			}
+		}
+		s.WriteRune('\n')
+	}
+	return s.String()
+}
+
+func (b *board) call(x int) (win bool) {
+	if b.won {
+		return false
+	}
+	for r, row := range b.nums {
+		for c, n := range row {
+			if n == x {
+				b.marked[r][c] = true
+				if b.win(r, c) {
+					return true
+				}
+			}
+		}
+	}
+	return
+}
+
+func (b board) win(r, c int) bool {
+	return b.winRow(r) || b.winCol(c)
+}
+
+func (b board) winRow(r int) bool {
+	for c := 0; c < 5; c++ {
+		if !b.marked[r][c] {
+			return false
+		}
+	}
+	return true
+}
+
+func (b board) winCol(c int) bool {
+	for r := 0; r < 5; r++ {
+		if !b.marked[r][c] {
+			return false
+		}
+	}
+	return true
+}
+
+func (b board) score(x int) int {
+	var sum int
+	for r, row := range b.nums {
+		for c, n := range row {
+			if !b.marked[r][c] {
+				sum += n
+			}
+		}
+	}
+	return x * sum
+}
+
+func part1(text string) int {
+	inp := parseInput(text)
+	for _, n := range inp.numbers {
+		for _, b := range inp.boards {
+			if b.call(n) {
+				return b.score(n)
+			}
+		}
+	}
+	return len(inp.numbers)
+}
+
+func part2(text string) int {
+	inp := parseInput(text)
+	var wins int
+	for _, n := range inp.numbers {
+		for i, b := range inp.boards {
+			if b.call(n) {
+				// Set on the original in inp, not the copy in b.
+				inp.boards[i].won = true
+				wins++
+				if wins == len(inp.boards) {
+					return b.score(n)
+				}
+			}
+		}
+	}
+	return len(inp.numbers)
+}
+
+func main() {
+	text := aoc.ReadFile("./input/day4.txt")
+	fmt.Println(part1(text))
+	fmt.Println(part2(text))
+}
diff --git a/2021/go.mod b/2021/go.mod
index f833a57..62552b8 100644
--- a/2021/go.mod
+++ b/2021/go.mod
@@ -1,3 +1,11 @@
 module github.com/jdkaplan/advent-of-code
 
 go 1.17
+
+require github.com/fatih/color v1.13.0
+
+require (
+	github.com/mattn/go-colorable v0.1.9 // indirect
+	github.com/mattn/go-isatty v0.0.14 // indirect
+	golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c // indirect
+)
diff --git a/2021/go.sum b/2021/go.sum
index e69de29..6ab7ee4 100644
--- a/2021/go.sum
+++ b/2021/go.sum
@@ -0,0 +1,11 @@
+github.com/fatih/color v1.13.0 h1:8LOYc1KYPPmyKMuN8QV2DNRWNbLo6LZ0iLs8+mlH53w=
+github.com/fatih/color v1.13.0/go.mod h1:kLAiJbzzSOZDVNGyDpeOxJ47H46qBXwg5ILebYFFOfk=
+github.com/mattn/go-colorable v0.1.9 h1:sqDoxXbdeALODt0DAeJCVp38ps9ZogZEAXjus69YV3U=
+github.com/mattn/go-colorable v0.1.9/go.mod h1:u6P/XSegPjTcexA+o6vUJrdnUu04hMope9wVRipJSqc=
+github.com/mattn/go-isatty v0.0.12/go.mod h1:cbi8OIDigv2wuxKPP5vlRcQ1OAZbq2CE4Kysco4FUpU=
+github.com/mattn/go-isatty v0.0.14 h1:yVuAays6BHfxijgZPzw+3Zlu5yQgKGP2/hcQbHb7S9Y=
+github.com/mattn/go-isatty v0.0.14/go.mod h1:7GGIvUiUoEMVVmxf/4nioHXj79iQHKdU27kJ6hsGG94=
+golang.org/x/sys v0.0.0-20200116001909-b77594299b42/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20200223170610-d5e6a3e2c0ae/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c h1:F1jZWGFhYfh0Ci55sIpILtKKK8p3i2/krTr0H1rg74I=
+golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=