Hi CentralPA,
It’s quite possible and it’s certainly a configuration I would try if 3 layers and a topper didn’t seem to be working out for you. The thickness of a soft layer (or soft layers) or of the mattress as a whole can certainly affect PPP as much as ILD and other specs that affect “softness” (see post #4 here and post #2 here) and thicker layers will “act” softer (there is more about the effect of thickness in post #14 here).
Of course as you mentioned all of this is theoretical and your own progressive testing will be able to “translate” all of this into your real life experience.
I have to say I’m impressed with your slow incremental approach and your patience!
You will be an “expert” in layering combinations by the time you’re done 
Phoenix