The most impressive thing about RE4, and what the sequels so miserably failed to recreate, was the progression of designed enemy encounters as you go through the game. In many action games before and after RE4, the developers seem to be struggling to find a gameplay loop that is fun for the player. If "something fun" is found, the instinct is to repeat the success over the entire game. Many successful action games are designed like this, and the best ones manage to use graphics, story, and positive feedback mechanisms such as collectibles to mask the repetition for the player. Another common strategy to mask repetition is to mix up the successful gameplay loop with scripted sequences that give the impression that gameplay is changing, when you are actually just pressing buttons to progress through interactive cutscenes.

On the other end of the spectrum, you have a few game series that are not designed with the described "repeat success" mindset. A particularly rare philosophy is when a developer sees a successful gameplay loop as a starting point, and tries to introduce new successful loops through an entire game. The "moment of fun" is explored and investigated from every interesting angle, but after that it is time to move on to the next thing. The problem is, developing games according to this philosophy requires really talented people, lots of time and huge piles of money. Nintendo is the the only reliable producer of these type of games, with the 3d Mario series and the Legend of Zelda games.

And that brings us to RE4. RE4 was developed with exactly the type of mindset that makes the Mario and Zelda games so unique. It was a revelation to play a third person shooter, or for that matter any type of shooting game, that explored enemy encounters to their fullest potential, but then briskly moved on to the next thing. There was no "trickery" to give a fake sense of gameplay progression. There were no "reskinned" levels with the same challenges. There was no overabundance of scripted sequences. There was no reliance on story to motivate the player to go through boring gameplay.

Instead of this, the game had a string of perfectly tuned enemy encounters, where almost every one brought something new to the table. However, RE4 did not just provide variation from one encounter to the next, but it also unified the encounters with a strong sense of progression, which it arguably did even better than the aforementioned Nintendo games. The increased player agency from finding weapons, pesos and buying upgrades were almost perfectly tuned against the challenge level in RE4, which made it so depressing to play RE5 (or RE6) where they just didn't care due to the impossibility of balancing coop anyway.