I played Civ IV, including a revisit recently. I like the diversity of options, and indeed the recent revisit prompted me to reevaluate that aspect relative to Civ V - and Civ IV is ahead by some way.
But diversity is not depth. Civ IV had very little strategic depth - there were two or three core strategies you could pursue, supported by optimal mixes of civics and optimal paths through the tech tree to unlock them and beyond,
and all but one of those ultimately boiled down to 'expand as fast as you can, choose a resource type per city and spam the appropriate +X% modifier buildings in it'. That's an exceptionally shallow approach to specialisation (Total War: Rome 2 did the same and it felt like a dumbing down even of the previous TW system, in a game series not historically noted for the complexity of its strategic layer).
Civ V ultimately failed in its go wide vs. go tall effort to vary playstyles; go wide was (almost) always possible, but go tall was the path of least resistance, and of lowest risk since early expansion required gambling on there being suitable city spots in sufficient quantity in your vicinity to be worth the effort. Sadly for a Civ game, going tall is also the path of least interest for a game of this nature. That was however a failure of implementation, not an effort to 'dumb down' - the developers wanted to permit more varied strategies to succeed than was historically the case in Civ.