If you have Pro Pack for 4, then it's a tough judgement call. If not, Poser 5 is now cheaper than adding Pro Pack to 4.

In Poser 5, you get:

• Hierarchical Library folders - very nice feature
• Firefly renderer + old renderer
• Pro Pack's Setup Room (for character creation)
• Pro Pack's Python scripting support
• Face Room - use photos for faces, limited
• Hair Room - only good for eating memory
• Cloth Room - cloth dynamics is a nice feature
• Content Paradise tabs - convenience links to Poser stuff
• Material Room using new node-based shaders - some love it, some hate it. :)
• No Pro Pack plugins available yet
• Some of the same bugs that have persisted for some time. You'll get them in Pro Pack as well.

So, it's really depends on what you need to do and how confident you feel about Poser 5's stability. The latest Service Release fixes many problems and removes the nasty protection system, so that is a consolation. Pro Pack, on the other hand, is rock solid. BYS