4 main dungeons seems pretty clear at this point. They obviously correspond to the 4 big mech bosses (lizard, bird, two more), and defeating each one will rather obviously lower/deactivate the 4 pillars surrounding Hyrule Castle. So your main quest is to finish those four areas and dungeons, kill the four bosses, and then enter the castle to face Ganon.
But I think the dungeons will have a lot of lead-up to them, similar to MM. And on top of that you've of course got the 100 mini-dungeon shrines. Plus side-quests, collectathons, etc. There will be enough stuff to do in this game. Personally, I'm actually rather happy that the main dungeons are kept at a reasonable number given how much else there will be to do.