I own them both. You own Civ 5 already. How do you like it? Are you considering buying Civ 4 in addition? I'm not sure what the question is, here.
The question is which is better: Civ 4 or Civ 5?
4 is the better game right now. Keep in mind that that game became great with updates and that it was not an instant smash hit. I still gave good hope that 5 will be very cool in the future.
But, very few people complained about the core game. Very few people wanted the old corruption system back, very few people claimed that the civics were bad and very few people thought that the new happiness/health mechanics were broken. Most people enjoyed the new combat rules, even though there were to few units in the game to make it really interesting. All Firaxis had to do was to fix the apparent to improve the game was to add more stuff.
But in Civ 5, people complain about the very core game. They don't like the global happiness, most of them agree that they AI can't handle 1upt and many don't like the fact that the game focuses too much on war. These problems can't be solved simply by adding more units.
Bad Brett has made the most compelling point in this thread - Very few complaints were made about the core game of Civ IV; Most of the complaints about civ V are about the core game. This truley summarizes the overall civ community's sentiment.
The hardcore fans certainly seem to prefer Civ 4, that's true, but I suspect for most players who just want a distracting, enjoyable and engaging game Civ 5 is the better choice.