Poser content does not normally require the install of any .dll files.  If you've downloaded some content for Poser that has .dll files, I'd be suspicious of it.  What are you trying to install?

As far as different versions, when content say's it requires a certain version of Poser, that normally means that it requires that version or any later version.  So, if it say's "Requires Poser 4" that means Poser 4 and up.

Unless it uses .bum files for the bumpmaps. (Poser 5 and below uses those, IIRC?)  Then, you have to convert those to jpgs by adding .jpg in place of .bum and correct the references in the figure/prop/pose files.