Cut the extra inch off the long side to get a 4" square, then cut the remaining 1" x 4" piece into 4 1" squares. The boy never said the squares had to be the same size.
If the triangles have already been cut, it's a peanut butter sandwich: use peanut butter on the edges to glue it back together and cut the squares. The child gave you a challenge, think outside the box!
If the triangles have already been cut, the kid gets a brand new sandwich fully intact, crust and all, and a knife. Let's see you cut this sandwich better than I can brayxtyn