Generally CA4 is backward compatible with content developed for earlier versions of the software from original CrazyTalk Animator (standard/Pro) and above. Although certain things like G1 and G2 characters will only work with features designed for them. For example you can't add a G3-360 head to a G1 character, or G1 and G2 characters cannot be exported into photoshop for editing etc.
G1 characters are most limited in that CA4 is only compatible enough with them so that they work and animate as you would expect using motion files specifically designed for them (you can't apply G2 and G3 motions to G1 characters for example).
I've been using CA4 since its first iteration so I have a lot of content released over the last 10 years that still works in the latest version. I do think there is going to come a time where Reallusion will just have to bite the bullet and remove some of the backward compatibility just to streamline the software.
- TET
Animation and Video Life
The Lazy Animator - Tutorials for Cartoon Animator