So those are two very different games.
Coming from Stellaris, EUIV would be the easiest to get into because it has a similar "nation-level" focus where you play an ambivalent national spirit that drives a country forward (like Stellaris). In CKII you have a character and that character's traits may limit the choices you can make. You also have to deal with inter-character issues, make sure you get your sons married off to women with huuuuuuuuuuuge... tracks of land, and avoid catching a rash when rolling in the hay with your rival's daughter.
If the character aspect sounds like fun, definitely go for CKII - Sword of Islam and The Old Gods are probably the best XPs for that and will open up Muslim and Pagan characters for play.
If you prefer the national spirit aspect of Stellaris, go with EUIV - As for XPs, not really 100% sure what the community considers "the best" but dig around and see which most impact the different game aspects you are interested in.