I’m not familiar with 6.0. Is that “6c3”? Since that was released a week after 5.0.3, it looks like Jive Cloud 6c3 is very similar to 5 but in the cloud, and having a few new features (https://community.jivesoftware.com/docs/DOC-58543).
If I’m right, I suppose that users would have similar issues moving from 4.5 to either 5 or 6c3. Let me know if 6.0 is separate from 6c3 though. It would be interesting to know how smoothly users adapted to either 5 or 6 (though my immediate concern is 5).
2. Was there any functionality present in 4 that was absent in 5 (meaning gone, not moved or renamed) that your users really missed?
Two things that I think experienced users in particular will react negatively to and therefore you should have good talking points to "spin" in a positive reaction.
1. No more Your View page
In Jive 6 there are some very nicely enhanced custom activity streams that I think address this for many users
2. No more Document, Discussion, Blog tabs at the top of the screen on your Overview pages for your spaces, groups, projects
These are replaced by the content tab, and then when you click on it you can get to those content types. It's an extra click - not a mile long hike.