To harp on one point, EU IV's religion system is more complex.

Civ IV treats religion as a set of passive bonuses with the ability to construct a special building or two, but the existence of differing religions really does not have any negative consequences. there is no real game mechanic that causes having multiple…

They are completely different genres. The Civilization games are a 4X franchise, where you snowball an empire from a single settler on graph paper. The Paradox games are sandbox historical simulators realized through grand strategy mechanics.

After putting a decent number of hours into both to call myself casually familiar with them, I'd actually say that Civ 4 is more complicated than EU4. It's really more about the look-and-feel of the games.