I was on the 1 for few years, and on 2 for many years.// but think 4 might be better just for faster focus..)
There's a lot of improvements in the 4 over the 2 in addition to the more sophisticated AF system.
I've found the following good reasons to be happy with the upgrade:
Built in GPS for geotagging
Quieter shooting (if you want it) or faster burst rates (great for hand held HDR).
More pixels of course.
Lower noise and better DR
Touch screen menu
SD cards are nicer to use than CFs, cheaper too.
Plus loads of other little tweaks and improvements compared to the 2.
The whole camera feels different, softer touch controls that initially felt rubbery, but once you've used it for a while going back to the 2 feels really 'clicky' and not so nice.
The only downside for me is the rather poor 4k video features, DCI not UHD and too much frame cropping. Frustratingly there was a strong rumour that this (and other improvements) would be changed in a firmware upgrade, but it's never happened :-(
Sadly I think you're correct in assuming that there'll never be a mk5, maybe the mk4 will be the pinnacle of true DSLRs.