I don't have Propack, so I can't test this, but I'd have thought it ought to be possible to install V4.1 into it, provided you are putting it into the directory that actually contains POSER.EXE.   You get the same message at any release of Poser, if you try to put V4.1 into a different directory.   V4 and V4.1 do, however, have features which aren't guaranteed to work at a lower level than Poser 6.