Hi Georgio!

 My understanding is that you definitely need V4.1 if you want to install the extra morph packs (muscle morphs and creature morphs), and since there are morphs in V4.1 that are not in V4 characters using those morphs obviously won't work.

Skin textures, clothing etc are totally compatible, unless the texture makes use of the V4.1 eyebrow prop.

When you install V4.1 (a more complicated procedure than normal- read the instructions), it does not overwrite V4, so you have the option of using either. The only major issue that was new to V4.1 that I encountered was the head/morphing tool one, and even a non-programmer like me managed to follow the instructions posted in the daz forum to correct that!

"Don't make me angry. You wouldn't like me when I'm angry!"