Never played any version other this Civ 5. If the reviews are true, Civ 4 must be the best game in the world because Civ 5 is pretty damn phenomenal.
*different kind of good*. Although, if CivV is your first in the Civ series, you'll probably be put off by Civ 4's stacks of doom, b/c they bog down gameplay and make it harder to figure out the state of everybody's armies. Plus Civ 4 square grid vs. CivV hex grid, and Civ 4 had more instances of older-era units taking down newer-era units that required extra imagination to keep the "narrative" intact (spearmen vs. tanks). Although I always laugh at continents completely built up with railways.
Your best bet is to try a Civ4 demo to get a feel for the controls, and then jump to the hour 20-30 of some Let's Plays and watch out for the highlighted criticisms. Personally, I'd probably just recommend you branch out and look at other 4X franchises or other strategy and complex sim games. CivV gives you the essential Civ experience already.