I agree with @Josh. From what I've read, upgrading from K 4.0 to K 4.0 should be fairly straightforward. As far as I can tell, K 5.0 contains one additional database table (someone can correct me if I am mistaken) and no structural changes to the other tables. This should mean that K 5.0 should be backwardly-compatible with K 4.0 and, if you find K 5.0 is not workable in your environment, you should be able to downgrade to K 4.0.
As far as the stability of K 5.0 is concerned, that's a matter of opinion. I agree with @Josh that there are issues with styling that affect the UX but I would add that there are bigger problems with the UI. However, given the history of development with K 4.0, there are a few question marks surrounding the stability of the feature set within K 5.0. Will new features be added that break parts of the system that previously worked; that kind of thing.
Unlike K 4.0 which effectively went through ten beta releases (during which time there were new features—some of which affected third-party template developers—added to the core) until K 4.0.6 was released, K 5.0 has been relatively more "stable". The bigger problems with the overall UX are, in my opinion, more to do with how users of Crypsis will adapt to the new appearance and layout of the forum controls. It is for reasons of the stability of the feature set, given the experience of K 4.0, that will prevent many people from adopting K 5.0 after the first "stable" version is released.
Overall, it would seem that should be few (if no) technical reasons preventing people upgrading Kunena.