Mini sudoku 4×4 (hard)

This is hard mode on the 4×4 board. The rules don't change — digits 1 to 4, once each per row, column and 2×2 block — but far fewer digits are given at the start. With fewer clues, cells rarely resolve from a single glance; instead you have to combine two facts at once: "considering both this row and this block, the 3 can only go here."

A 4×4 grid still takes only a few minutes, which makes this the right level of resistance for a child who has finished easy mode or an adult who wants a short mental workout. Every puzzle is generated with exactly one solution, so it always yields to reasoning rather than guesswork. If you get stuck, Check highlights wrong digits in red, and Show solution reveals the answer.

Once 4×4 hard stops slowing you down, the wider 6×6 mini sudoku is the natural next size. For the full game, the 9×9 easy sudoku gets 20 new puzzles a day. Techniques are explained in how to play sudoku; all mini sizes are on the mini sudoku page and every level on the home page.