New lighting features have been added. Poser 3 has only three lights. Their direction, color, and intensity are controllable. Poser 4 allows an unlimited number of lights which are easily turned on and off. And they can be changed from flood lights to spot lights, another feature the lack of which stymied me in Poser 3.
Morph targets allow body parts and props to change to completely new shapes.
Most of my previous animations have been of ballet figures. When I heard that Poser 3 did facial expressions and even had built-in expressions for specific phonemes, I decided a change was in order. As soon as I started my Diva's animation, I realized I was in trouble. None if these is suitable for my Diva on the opera stage. So I resigned myself to doing and head and shoulders only animation, very limiting cinematically.