To me, 4 was the pinnacle. It was hard, but you could basically work your way through it, and it was so rich that there were times playing it that my brain would hurt trying to plan what I wanted to do in the game.
5 had much much much less complexity. When it came out, I was disappointed. But then I went back to 4, and could never play it again. While 5 certainly did not have the richness of the past, the hexes, 1 upt, trade routes, ranged combat, etc... version just made it impossible to play the old version. It certainly didn't challenge me nearly as much, but it was still certainly a very fun game when you just wanted to go out, build some stuff, and go conquer the world.
Now oddly enough, I'm almost encouraged that 6 doesn't seem to be a radical departure from 5. That gives me more hope that they will have spent more time focusing on the AI, and ensuring that all the game elements are truly balanced and usable. I like how they seem to be encouraging more terrain-based strategies. I like how they seem to be somewhat moving back to the city specialization of 4, which was really the best part about it. In a lot of ways, I feel they're bringing back the better elements of 4 while keeping the general structure of 5. So I actually have good hopes about it.
I'm not going to pre-order, yet. I still haven't seen quite enough to make me jump on it. But I feel there's a lot of potential here for a release that could rival 4.