I think @impynickers nailed it. RE4 basically began the current crop of third person shooters we see today, even if only indirectly by way of inspiring Gears.
However, even if other games have improved the controls and base mechanics, I think RE4 remains unsurpassed in the areas of game balance and level design. The game still manages to have some of the most intense set pieces I’ve ever been through. When I say “set piece” I don’t mean cinematic QTEs that a game basically steers you through, but actual level design setups perfectly planned to stress your resources and strategic reasoning. That whole thing became a masterpiece on professional mode. The game does have a lot of QTEs though, but I think almost none of them actually cut you away from gameplay.
Also, don’t even really expect a legitimately creepy horror storyline or any of that shit. It’s the only RE game that knows how ridiculous the franchise is and plays that up like a motherfucker, complete with Schwarzenegger-esque quips. To compare it to Evil Dead would be an overstatement. For now let’s put it somewhere between Evil Dead and a self-aware version of The Rock.