Yea, the battles in Civ4 are frustrating when a iron age unit defeats a modern unit.
This never bothered me. Consider this:
The guy in the tank has this really slick armor system. The tank. The Iron Age guy...doesn't. But the guy in the tank can't spend his entire day in the tank, he has to eat, sleep, wash up, spend time with the girlfriend, do his taxes, etc... Once he's outside of his tank, he's just a guy. And if the Iron Age guy (who is also a guy and knows something about where to stick the pointy bits into another guy to cause death or other fatal injuries) can get to the tanker when he's not in the tank, he can "destroy" the tank by killing the guy who drives it.
Unlike what we see in the tactical viewer in GalCiv II, or the combat sequences in SoaSE, or the sometime-to-be-replaced text lines in Elemental, the Civ games have always been abstracts of the combat and not the details. Truly that's the only way they can account for the amount of time it takes one unit to destroy another unit early in the game, when turns span decades.