Create iMovie cloning of myself for MORE than 2 videos

I know how to create a clone of myself playing two instruments on stage to the same tune. It looks like I have a twin. The problem is creating a video with me playing 4 instruments on stage at once.

I import 4 videos.

The first two go in easy enough with me selecting the second "me" video I drag and put on top of the first "me" one,

I hit the square icon (video overlay settings) then select "green/blues screen" from the drop down.

I can crop the second movie to reveal part of the first "me"

If I try and pull down a 3rd video of "me", it will not go on top. It wants to push the second one away or slide next to it.

Love to have this for many of my adult musician students. Mac Sequoia 15.5 iMovie 10.4.3