Working Through Four 9x9 Grids
On each grid, remember that a row, a column and a 3x3 box always total 45: a cell that the cages of a box leave uncovered is 45 minus their sums. Open with the cages that have one combination, two cells of sum 17 holding 8 and 9 or three cells of sum 6 holding 1, 2 and 3, and keep candidates to the corners of the small cells.
Three Levels of Difficulty
On easy grids, eight digits are written in advance and the cages are short, a gentle start for a first 9x9. Medium grids start empty, with cages of two and three cells. Hard grids add cages of four cells, whose sums allow more sets of digits.
When to Choose This Format
Four 9x9 grids per sheet make a dense set for a long journey or a solving session, with less paper than four separate pages. For larger cells, choose the 9x9 Killer Sudoku page, with one grid on each sheet. Both formats come out on A4, in black and white.