I've just upgraded my mark ii with the mark iv. The main motivation was video - to get 4k and slow motion.
I've only played with it for a day, but aside from the obvious difference in video capability the main difference seems to be processing power. Everything's just a little snappier and quicker. Time will tell if there's much difference in the image quality, but I was pretty happy with the images I was getting from the mark ii anyway. Reviews seem to suggest that high iso performance is improved.
Upgrading it has made me have a wider rethink. I realised that I've really only seriously used my D750 twice in the past 18 months, on both occasions for F1 testing. In between I've only used it occasionally, and then in situations where the RX100 would still have done a decent job. So I'm now thinking of selling the D750 and lenses too. I love it, and it feels like tough decision, but thinking rationally I'm simply not using it enough any longer.