The Civ5 graphics are definitely worse in a technical sense. The game runs like molasses.
I dislike the way that some developers seem to think high system requirements show that a game is "advanced" and are to be worn as a badge of pride.
No! A game is advanced if it runs smoothly on any system (and don't need a DX11 graphics card just to enable anti-aliasing). Microsoft realised this for Windows 7 after the Vista debacle.
Functionally, Civ4 wins hands-down, since you can clearly see what's going on.
Subjectively, I don't think Civ5's realistic water effects and fog, which are the main things that are different, add anything to the game. Civ4's wonder videos did add a sense of achievement, as well as atmosphere to the game, unlike Civ5's crap pictures. But I might just feel like this because Civ4 is fun and Civ5 isn't.