Dominions 4 isn't in the plans. Illwinter is working on
something, but the only thing we know is that it isn't Dominions 4.
One of the reasons for not working in a sequel to Dominions is that they would have to do everything from scratch. The current engine is too old and not flexible enough. There's not much new they can do with it. As an example, modders have noticed that there are only few hundred free slots for new spells or magic sites, which means that conflicts between mods are quite likely, and that complicated spells with many different effects aren't feasible because they take several spell-slots.
Any way, if we were to have a new Dominions engine, I'd like to see more modding, but I'm not sure what else. Better user interface, perhaps. Hmm. A campaign editor for single-player, that would allow you to set events and things that'd happen at spesific turns, as well as story-messages keyed to different events and ability to display pre-recorded battles. It'd be something like the current megamaps and such (Antilarium and that huge one with a dimension for each distinct magic), except with plot.