It's definitely true that P4 body morphs don't work on P3 figures. Poser refuses to apply them, coming up with the "different number of vertices" error. That's why Poser 4 comes with a set of P3 nudes. However even the P4 versions of P3 figures aren't exactly the same as the genuine figures that come with Poser 3 - although they take the morph targets, they won't accept UVmapper .UVS files meant for the other figure. There is some difference in the UV mapping.