The difference between Dom3 and Dom4 is much like that between 1 and 2, I think. I rather enjoy it quite a bit, and I think it was worth the cost, but I can see where some might disagree. Some things you get in Dominions 4 that are new:
1. New game type: Disciples. Essentially, multiplayer teams where one player plays a Pretender God and their teammates play the disciples of that God, and all team members win together.
2. Thrones. Replacing the old Victory Points, Thrones are in-game sites that have tangible effects ranging from unique commanders, gems, dominion spread, bless bonuses, and events.
3. New nations and pretenders. Some of these are old Dom2 themes given their own nation (Scelera and Asphodel), but many are new.
4. Tweaks and revisions to reduce micromanagement. Examples include displaying mage paths right on the unit display (no more renaming mages stuff like "F2S2W1N9") and the removal of the tax scale.
5. New map features, including mountain passes (passable only when the snow is melted - both sides have a Heat scale) and rivers (passable only when frozen - both sides have a Cold scale). Movement rules were also changed, which makes terrain much more important strategically.
6. Research rebalancing. Rather than racing straight into the late game, research is now scaled exponentially to encourage the use of national armies for longer.
7. Slow-to-recruit units. New feature to rebalance certain powerful mages or commanders, which now require two turns to recruit. This also encourages the use of cheaper mages that can be spammed every turn.
8. Gem refiners have gone the way of the dodo. Now, so they are used the way they're intended, they work to produce temporary gems only in battle. See 4.
EDIT:
Oh, I entirely forgot...
9. Repel. Spears and weapon reach now work defensively as well as offensively - if you're holding daggers and facing a pike wall, it's going to be pretty hard to get through to stab someone. This makes spears a bit more useful, and makes for a better balance between whether you need the sheer killing power of swords or axes versus the stand-off ability of spears.
EDIT 2: And I should probably finish my sentences before I get distracted by- ooh shiny