Johannes
Arcane
Off the top of my head, it added slow-to-recruit mages instead of everything recruiting in 1 turn, removal of reverse communions, thrones as mentioned, disciples mode, new spells and nations, UI improvements, upgradable forts, gemgens don't give permanent gems and hammers give fixed bonus instead of percentage, death upon retreating in fort defense, had to have a line to a fort to draw income from a province, and lots of small things like that.
4 I dont remember what changed compared to 3.
The change from turnbased to realtime battles was the big change in 4 -> 5, maybe bigger than anything in 3 -> 4.