2-cell killer cage summing to 4

A 2-cell killer cage of 4 is fully locked: the only way to make 4 from 2 distinct digits is {1, 3}. Whatever order they sit in, those digits are the cage's contents — every one of them eliminates from every cell of the cage's row, column, and box that lies outside the cage.

Locked digits: 1, 3. 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.