1. No, she can't.

V4 is new geometry, new joints, new everything, so she needs completely new clothes and textures.
There is a V4toV3 legacy figure included that can wear V3 clothing, but it can't use bodymorphs so you're stuck with the default V3 bodyhape when you use it.

2. The shoulders are better, but have VERY high armpits.
   The rest bends not better than V3.
   (Actually, the thighs bend WORSE because V4 lost the separate "buttocks" group that V3 had)

To make the shoulders look better, DAZ added Joint Smoothing Magnets to V4's cr2.
These 69 JSM magnets add 207 additional actors to V4 which slow Poser down and make V4 incompatible with a lot of other programs and python scripts that work flawlessly with V3.
There are also numerous other problems with the way V4 is constructed.

Generally speaking, she is way too complicated and tends to break under her own weight.
V3 is much more robust.

If you can afford Poser 7, the built in MorphBrush can easily correct all of the older meshes shoulder (Or ANY other) problems, so there is basically no need for V4 at all.

Here's a small example of what the P7 MorphBrush can do:

Shoulders WITH P7 MorphBrush morphs:

Shoulders WITHOUT P7 MorphBrush morphs:

Straight P7 renders with NO postwork.