One thing I will give Poser credit for, it is very good about forwards and backwards compatibility. It'll just ignore the material nodes; it won't be able to use them, but it won't choke on them either. Usually I package the bump map in jpg format and two different material poses: one calling for the jpg and one calling for the same file name except with a .BUM extension. Then I include a note in the readme for P4 users to convert the jpg to BUM in Poser before using the MAT. A lot of items do it that way, so most people understand it. (Or at least know what to mutter curses about if they forget to convert. ;) )