I prefer Civ 5 for the interface alone. It's just easier to see what is going on.
Civ 4's map and technology trading, local happiness and health, number-valued diplomatic modifiers, these are some of the few (dwindling) redeeming qualities about that game. I'm sure you'll get a lot of arguments about one unit-per-tile, but that's only because there are two opposing views on whether turn-based combat should be either strategic or tactical. Stacks or carpets. In Civ 4, it didn't matter what kinds of units you had so long as there were many of them produced quickly by your bigger cities and joined in a single tile, adding to their overall "toughness". In Civ 5, it was important to have a nimble yet diverse army with units that performed some functions better than others, rock-paper-scissors style, with moving and positioning becoming much more vital.
That's not at all how civ IV combat worked. The bulk of your army needs to be siege units, but you also needed a couple of stack protectors and mop up units. You're going to lose every war you fight on noble and above if you just spam axemen. How exactly is that any worse than spamming ranged units because they're a lot better than any other unit?
It's so annoying when the Civ V haters insist that everything about Civ IV is superior. When you ask them if they've played BNW yet and they answer "no", you just have to shake your head.
The removal of stacks of doom and use of hex tiles is enough of an improvement for me.
Do you seriously expect people who hate Civ V to buy BNW on release? BNW really doesn't address Civ V's main problem either, the game is flat out too easy. Everybody and their mother plays deity, and as of right now BNW makes deity even easier.
*sigh* My point is don't judge a product (especially with the recent BNW) without playing it. In this case people need to stop acting like Civ IV was god's gift to gaming and stop trashing Civ V over trivial stuff.
Civ IV was just as stupid when it was first released. It wasn't until BtS that things got better.