Go for it!  I've been running for years with both P6 & P4 installed on the same external drive with no problems at all.   None of Poser is on my C: drive.  It is a desktop XP system, but unless the laptop presents some quirk you should have no troubles.

A little overstatement perhaps, but really, the apps themselves run fine.  The only hassle I had was when initially switching over, some figures & props (just a few) had specific file path references to texture maps located outside Poser's Textures folder and those had to be changed to the new paths.   The "Correct Reference" app came in handy there, and  IIRC, I stuck in a few shortcuts in the old locations on the  C: drive that point to the new locations on the external drive for any specific file references in old files I failed to change.