The only problems are in (compiled) Python scripts. Everythings else working in any version of Poser will work in any newer version of Poser (the other way doesn't always work, although most can be made to work in a way).

Anything specifically made for daz3d won't work in Poser (as there are differences in materials and some other things). Most things work in both applications but some things work better in one or the other. In general all paid for products clearly indicate what works and what won't. The free products might work (but that even applies to the application it is intended for).

Specifically all 1 to 4 Daz products work flawlessly in both applications. With Genesis (5) Daz introduced new features that didn't work initially but they seem to have overcome the problems (I'm personally not yet convinced). Daz has woken up to the fact that a large portion of their customers is using Poser and will continue to do so.