It is hard to rate PDX games difficulty, as they usually don't have defined victory conditions and the main difficulty is in starting with a weaker ruler/nation. I'll grant you that Vic2 was a harder game than their current new releases, but I would not say it was harder than civ4. Civ4 has one of the highest skill caps of any game, that's why some of us are still playing it
PDX games in general are known for appearing complex but the complexity results in fewer interesting decisions not more. For an example, in EU4 combat works with a series of dice rolls, 3 phases of combat, defensive terrain, river crossings, unit perks, the stats of the general, the traits of a general, unit buffs, unit movement speed, reinforcements, global modifiers, combat width, flanking damage, attrition, morale, etc., etc. The result? Well there are only 3 types of land units: infantry, cavalry and artillery. Cavalry is not worth it. Artillery becomes OP late game. If that sounds bad, well naval combat is even worse. This is super pathetic. The complexity of PDX games is a flaw that drains developer time and unintuitively results in their games being much simpler as far as decision making goes.
PDX games have certainly gotten dumbed down, but I think a lot of this has to do with title spam and DLC spam, forcing them to spend all their resources on creating new screenshots for the latest steam page. The Civ series has no such excuse. They've been handicapped immensely by 1UPT, and now they've got a fanbase that would revolt if they're not able to feel like geniuses for tactically destroying a braindead AI.