Once I learned to use P5 there was no going back. I only have Poser 4 installed because I have to make sure MAT Pose Edit is compatible with it.

Right now most of my projects use P5 features so heavily that it would be almost impossible to create the same for the old limited Poser 4. My modified P4 Man for example (attached image, still a WIP) uses displacement mapping for the geometry details, which means fast rendering, and no need for high-poly models. I couldn't get the same result with morph targets.

If you're animating characters with clothes on (yeah right, like anyone would do that!), you'll love dynamic clothing. You can convert most conforming clothes into dynamic ones, and new clothes are easy to create because no frustrating poserizing is needed... and you can make most pieces of clothing fit many different characters. I don't have "Vicky clothing", "Judy clothing", "Mayadoll clothing", etc. in my library, just clothing.