There are only twelve 4x4 sudokus (and a cool trick for finding minimal subsets)
Researchers have proven that only twelve fundamentally different 4 × 4 Sudoku grids exist when accounting for rotations, reflections and digit relabeling. The study also outlines a simple technique for locating the smallest sets of given numbers that still guarantee a unique solution, offering insight into minimal clue configurations for these miniature puzzles. The enumeration used computer search, and the trick exploits linear‑algebraic properties of the matrix.