Split Screen – To heighten a two-person scene, Player Three and Player Four initiate a new scene – on the same stage, but existing in separate physical spaces. For example, a scene about a married couple fretting over money can be heightened by a couple of mice fretting over cheese.
These two (or more) separate scenes can continue at the same time (usually on opposite sides of the stage), sharing focus back and forth. While they do not exist in the same physical space, information from one scene affects the other as the focus shifts.
Or… The original players can fade off stage as the second set of players establishes their scene, and this second set can fade off as the third set establishes their scene. This is especially useful with smaller numbers of players in a group and can allow themes to heighten faster with subsequent iterations.