No problems in any of these areas. So far as I am aware there are no backward compatibilty problems between ProPack and P4.

ProPack doesn't need to use .bum files, it can render bump mapping straight from a normal (.bmp, .jpg etc) file and doesn't perform a conversion. If, however, a .bum file is specified in the material, ProPack will use that in the same way as 4.03.

I think (but can't remember now) that ProPack converted all of the library .rsr files into .pngs on installation. Either way, I now have library directories that have both .png and .rsr files for the same objects, Propack seems happy using either. The main reason that a lot of people delete the .rsrs is simply to save disk space.

The only downside is that if you are distributing objects from a ProPack installation you may have to convert the .pngs to a .rsr format in order to allow P4 users to access them.