I think it Civ V is a better game, it's just a ton of fun. I love the hexes, I love the combat (ranged, 1UPT), I love the social policies, I love the music mixes, the leader screens are awesome, natural wonders are great, city-states always provide an interesting challenge and I think having truly unique civs are strong points. The great wonders are much better balance now too, every wonder is worth prioritizing based on your strategy whereas in Civ IV some like Pyramids and Great Wall were simply OP, while others like Chichen Itza were practically useless.
Civ IV had some things better, however. First, the Diplo AI was more complete. The AI behaved in a rational (albeit not always competitive) way, you could make and keep friends all game. Civ V really needs a Diplo AI overhaul. Further, AI combat is about the same in both iterations but combat in Civ IV was steer a large group of folks toward the nearest civ, see who had the most catapults, and good luck. Combat in Civ V is so much more in depth that it needs an AI that can really evaluate and make good strategies, and that is where it suffers. Modding was much easier in IV as well, all of the source was released and simple programs like Nifskope could create new buildings/units that would work in game. The tech tree in Civ IV made more sense, it allowed for limited bee-lining but understood that you would of necessity know how to make a train before a mechanized infantry, or having Infantry without ever learning "Rifling" was impossible. Finally, the loss of espionage is a serious detriment, you could do a lot with a good espionage system, especially since tech trading is out, so stealing a tech could be huge.
Finally, I preferred the cottage system in Civ IV to the trade posts, the upgrading nature coupled with the little towns dotting my landscape was nice to look at. However, roads are much, much more important and better done in Civ V.