There was a Python script called PuppetMaster that would convert poses (still and animated) between different figures, but has needed (I believe) recompiling for Poser 7, and e-frontier never got round to releasing the updated version. Hopefully SmithMicro may get around to it.
Failing that, if you are using Poser 7, it may be worth nipping over to PhilC's site in this thread, http://www.philc.net/forum/viewtopic.php?t=1488 and installing the V4 dat files into P7, saving the V3 pose as a universal pose, and try applying the newly saved universal pose to V4. Other than that it's just a case of applying the V3 pose and tweaking until it looks right.