I have enjoyed both IV and V equally preferring different things in each game.
It drives me crazy having a nice bit of land to build a city in V and not being able to because of happiness. If there was a nice spot to settle in IV, you settle there. I miss that. I also dislike having to build, say, libraries in all cities to build National College. I prefer how in IV you have to build a set number of buildings, not one in every city. Obviously, I'm a much greater fan of going wide, not tall, and this is favored moreso in IV.
However, replayability is far greater for me in V as different civs play completely differently. In IV, I was doing the same thing every single time because the goal was the same no matter the civ you played. In V, playstyles vary so much amongst the civs that if you get bored playing one way, switch to a different civ.
All things considered, I'd never go back to IV. I played it a ton and its time has passed. Likewise, when VI comes out, it will be time to retire V.
I know what you mean. I've probably racked up the most hours on Civ 2, but I'd never go back to it now.
I sometimes go back to IV to play Fall From Heaven, but not for any other reason.
For me, its not that V is objectively better than IV, its just that V is the current shiny lovely game, so thats the one I play.
If I were going to pick one thing I loved about IV way more than V, it is that when you click "end turn", the next turn starts instantly.
Mind you though, this thread is making me nostalgic for it. I may just play a game of Civ 4 tonight.