People around here who seem to know seem to think that Dominions 4 isn't coming, or that if it IS coming we won't see it for a long time. The developers' newest game COE3 ( http://illwinter.com/coe3/index.html ) is coming out and they probably aren't working on another huge game at the same time. If you're interested in Dominions, definitely get Dominions 3.
I was in the same situation 6 months ago and i don't regret my expense.
There is still a lot of newcomers and the community is bigger than what you can see here. I'm ready to bet that this game (his community) will be still alive in several years.