Civ 5 is overrated, especially when compared to Civ 6. The DLC for 5 is what made it an even passable game. In Civ 6 you get everything in Civ 5 off the bat without DLC. 

Civ 4 is oddly underrated and I actually like it over Civ 6. Though Civ 6 for me is the perfect mold of tactics and strategy. Civ 4 is just very deep I can see why it can be offputting apart from the dated graphics. Some things are just ridiculous like vassel states. Not only will they help with war but they automatically vote for you in a diplomatic victory. The fact Wonders go obsolete also bugs me. However it's probably the deepest one when it comes to AI and how you can balance some things diplomacy wise. Especially religion and how some AIs won't even attack you if they're friendly with you. Edit: Plus while it takes a while to unlearn the sprawl and crawl battle tactics of Civ 5, Civ 4's doom stacks are far more classic and kinda what I prefer.