The CKII kingdom (let's say England) can use every CKII mechanic on itself and on the EUIV kingdom (let's say France), and vice versa. Armies are of equal strength at the game start, which is 1066 for CKII and 1444 for EUIV. They share a land border (England has Normandy) but otherwise there's nothing but sea between them. England is the attacker (Claim on k_france, aka cores in all of France). Feel free to speculate for the other factors.
Imho, CKII-England stands a good chance at the start because their armies can land anywhere (EU4 ships can't interact with them) - plus they can assassinate generals and good rulers. But when EUIV-France invents artillery, England is toast.
Imho, CKII-England stands a good chance at the start because their armies can land anywhere (EU4 ships can't interact with them) - plus they can assassinate generals and good rulers. But when EUIV-France invents artillery, England is toast.