4-cell killer cage summing to 12

Only 2 digit sets sum to 12 across 4 cells, which makes this a near-locked cage. Each combination shares most of its digits with the others — see the locked-digits row above for the digits forced into the cage regardless of which combination is in play.

Locked digits: 1, 2. Whichever combination is in play, these digits sit somewhere inside the cage — meaning the cage's shared row, column, and box can't have any of them anywhere outside the cage.

Forbidden digits: 7, 8, 9. These digits never appear in this cage no matter which combination resolves; you can eliminate all of them from every cell of the cage as you pencil in candidates.