To my best understanding legacy content has always been available with each progressive version of Poser. In some versions it can on the accompanying CD in others it was offered as a free download.

The Poser 2 female has the same body parts and hierarchy as V4. Poser 7 and Poser Pro have the universal pose feature that will take into account variations in the zero poses of each figure making it possible to accurately transfer poses from one to the other.

V4 has 66,830 polygons
The P2 figure has 2,389 polygons
Smoothness of the animation scrubber will be in proportion to the number of polygons, particularly if a display mode is chosen that does not require textures.

When I need to create an animation I use the low polygon figures then transfer the motion over to my final figure prior to rendering.