diff --git a/day4/example.txt b/day4/example.txt
new file mode 100644
index 0000000..8209399
--- /dev/null
+++ b/day4/example.txt
@@ -0,0 +1,10 @@
+..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.
diff --git a/day4/main.go b/day4/main.go
new file mode 100644
index 0000000..7d063bb
--- /dev/null
+++ b/day4/main.go
@@ -0,0 +1,100 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"log"
+	"os"
+)
+
+func main() {
+	f := os.Stdin
+	if len(os.Args) > 1 && os.Args[1] != "-" {
+		var err error
+		f, err = os.Open(os.Args[1])
+		if err != nil {
+			log.Fatal(err)
+		}
+		defer f.Close()
+	}
+
+	grid, err := readDiagram(f)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	var total int
+	for i := 1; ; i++ {
+		removed := removePapers(grid)
+		total += removed
+		fmt.Printf("%2d: removed %d\n", i, removed)
+		if removed == 0 {
+			break
+		}
+	}
+	fmt.Printf("total %d\n", total)
+}
+
+func removePapers(grid [][]bool) (removed int) {
+	type position struct {
+		x, y int
+	}
+	var toRemove []position
+	for y, row := range grid {
+		for x, b := range row {
+			if !b {
+				continue
+			}
+			if countAdjacent(grid, x, y) < 4 {
+				toRemove = append(toRemove, position{
+					x: x,
+					y: y,
+				})
+			}
+		}
+	}
+	for _, pos := range toRemove {
+		grid[pos.y][pos.x] = false
+	}
+	return len(toRemove)
+}
+
+func countAdjacent(grid [][]bool, x, y int) int {
+	var count int
+	xFrom := max(x-1, 0)
+	yFrom := max(y-1, 0)
+	yTo := min(y+1, len(grid)-1)
+
+	for yi := yFrom; yi <= yTo; yi++ {
+		xTo := min(x+1, len(grid[yi])-1)
+
+		for xi := xFrom; xi <= xTo; xi++ {
+			if xi == x && yi == y {
+				continue
+			}
+			if grid[yi][xi] {
+				count++
+			}
+		}
+	}
+	return count
+}
+
+func readDiagram(r io.Reader) ([][]bool, error) {
+	var grid [][]bool
+	sc := bufio.NewScanner(r)
+	for sc.Scan() {
+		var row []bool
+		for _, b := range sc.Bytes() {
+			switch b {
+			case '@', '.':
+				row = append(row, b == '@')
+			default:
+				return nil, fmt.Errorf("read diagram: unexpected character: %q", sc.Bytes())
+			}
+		}
+		grid = append(grid, row)
+	}
+	return grid, sc.Err()
+}