The HW 5 installer replaces HW 4 if present. About the only ways to back out of a trial on the same machine would be a full disk image backup and restore, or possibly uninstall HW 5 without migrating licenses, install HW 4, and restore your user settings backup and reinstall your samplesets. I may be missing some potential hazards there.
You don't have to clear out old cached organs or uninstall V4 organs before installing their V5 versions. HW 5 will recache every organ when you load it. Reinstalling the samplesets overlays their data and organ packages.
The only cleanup I had to do was to delete V4 RAR files when I got V5 RAR files for that particular organ, to save space. The V5 installation RAR files are named differently from the V4 files they replaced, at least for the four producers' files I've downloaded so far.
Don't reinstall V4 organs that weren't encrypted or dongle licensed -- those work in V5.