I don't think there would be any insurmountalle problems with the textures in P5, some material nodes may not work, but you should be able to achive something that looks good with a bit of tweaking.

I think a bigger problem will be the version numbber used in V4. V4 uses a version number of "6". V4 also uses 'readScript' lines to apply a large number of pose filed to load morph channels etc. I think that means that you will get an error message, something like "The file is a newer version, Poser will attempt to read it anyway", then you have to click "OK" every time Poser tries to load one of the files. After you have loaded V4 once, you can save a copy to a pallet, using that copy you won't get the error messages, but you might have to go through the same procedure if you install a new morph pack.