can they still use figures for P4 and P5?

P1 and P2 could use no external figures at all - P1 was entirely limited to figures and props that were built-in, and P2 only allowed to import (static) props but no figures.
Because of that, you can neither use P3+ models with P1 and P2, nor can you directly import the original "built-in" P1 or P2 models in P3+.

The only files that survived the P3 revolution are textures and pose/lights/camera library files. In order to keep them working, P3+ included the "P1" and "P2" models, but these are actually "emulated" models - they are new P3 models that behave and look like the original P1 and P2 ones (to allow to use the old textures and poses).