4 cameras wouldn't work very well with the cameras I used. It was a struggle with 6 cameras to find enough overlap get good quality stitching.
One thing that would be great is to get cameras that can be arranged in a tight enough configuration that they all share the same non-parallax point. Then, you don't have to do anything fancy to stitch them.
One piece I kind of glossed over is that at lower altitudes the ground is much closer to the camera than the sky. That means that you can't create a general case stitch profile as you're not working from the center of a sphere (in relation to the objects you're trying to use for control points), so as the payload rotates you have to stitch each frame individually. I may play at it some more and try to get a stitched video of the low altitudes of the descent. First, though, I'm going to take a break from the project for a bit.