I have both Poser 4 Pro Pack, Poser 5 and Poser 6 (and Daz Studio) installed on the same HD. Not in the same folder of course, but P5 and 6 both resides in a Curious Labs folder. No problems at all.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.