If you run the DAZ installer, it locates the Poser(TM) 4 folder; but the popup allows you to select another directory. You can select the Poser 5 folder (um, right now I can't remember if you select the Poser 5 folder or the Runtime folder inside it, but the experiment is easy), and it'll install there.

You can also use your Poser 4 runtime from Poser 5, if you want. (I chose not to, just because I had so much miscellaneous junk there.) I copied a bunch of "obvious" stuff from the Poser 4 runtime; though it's sometimes tricky because you have to find the geometry, pose files, and textures. (If you don't want to use the whole runtime, it's probably easiest to find the original zip or installer file.)