X Sudoku 4×4 Medium — Set 51
Solver Notes
A 4x4 X-Sudoku at Medium difficulty. Fewer given digits mean you will need to combine diagonal and box reasoning for some cells.
X-Sudoku — Free Online & Printable
X-Sudoku adds a diagonal constraint to the classic sudoku rules. Both main diagonals — running from top-left to bottom-right and from top-right to bottom-left — must each contain the numbers 1 through 9 without repetition. This is the most accessible sudoku variant: the two extra constraints actually make many cells easier to deduce, since numbers on the diagonals are additionally restricted. If you've mastered standard sudoku and want a fresh twist without learning complex new rules, X-Sudoku (also called Diagonal Sudoku) is the ideal starting point.
About 4x4 Grid
The 4x4 grid is the introductory format for variant sudoku. With only 16 cells organized into 2x2 boxes, a 4x4 puzzle takes 30 seconds to 2 minutes to solve. At this size, the variant rules (diagonal constraints, window regions) are immediately visible and easy to apply — perfect for understanding how a new rule changes the solving experience before tackling larger grids. The 4x4 is specifically recommended as a first puzzle when learning X-Sudoku, giving you a low-stakes environment to understand diagonal constraints without the cognitive load of a full 9x9.
Medium Difficulty
Medium difficulty requires applying variant rules in combination with standard techniques. You'll need hidden singles (where a number can only go in one cell within a region, even if multiple numbers are possible for that cell) and basic elimination chains. For Killer Medium, expect some cage combinations where two or more possible sets remain and must be narrowed by row/column logic. Average time: 10-20 minutes for 9x9. Candidate notation is helpful but not essential.
How to Get Started
- Choose "New Puzzle" to generate a fresh puzzle with a unique solution
- Click or tap cells to enter numbers (1-4 for 4x4)
- Use the constraint highlights to track which numbers are still available
- Print the puzzle if you prefer solving on paper