I took a look and as far as I know, you don't get anything depending on your rank in Resident Evil 4 (There is actually no ranking system in Resident Evil 4 as far as I can remember besides in Mercenaries.), however you do get something in some of the other games. Resident Evil 0's ranking system is only determined by how long you complete the game after wards it unlocks Leech Hunter and the Closet Key when you beat the game, however if you get an A (Complete the game in at least 5 hours) you get a submachine gun and getting an S rank (By completing the game in less than 3 and a half hours) grants you a Rocket Launcher.
Resident Evil 2 also has a ranking system and it is determined by how long you complete the game, how many healing items you used and what kind of healing items you used, and rather or not you used a weapon upgrades or special weapons. In that game you at least need a B rank to unlock something, but if you want to play as Hunk's mini-game then you need to have an A rank on both Game A and Game B. There is an unlockable Tofu game, but I honestly don't know how to it, I've been hearing "Get an A rank in 6 scenarios", but I have yet to play that game yet so I don't know what a "scenario" is in RE2.
Resident Evil: Code Veronica also had one and I do not know how it determines your rank, but I'm hearing that it is based on how long you took the game, how long it took you to save Steve at the beginning of the game, rather or not you gave the medicine to Rodrigo, how many first aid sprays you used, how many times you saved, and how many times you retry. I also been hearing that it uses a points system and for every "flaw" you make (Like saving) then you lose points and you can't regain them, but I have yet to beat Resident Evil: Code Veronica as yet so I can't say anything on how the ranking system works. If you want to unlock something in that game you'll need to get an A rank in main game and you're reward is the zombie bane (Aka the Rocket Launcher).
Resident Evil 5 and 6 also have a ranking system and they are pretty much based around the same for each other, it all depends on how long it took you to beat a level, how many enemies you killed, how many times you died during a chapter, and maybe one or two more, but I don't know. As for those games, Resident Evil 5 does grant you some figurines depending if you get an A or S rank while Resident Evil 6 gives you nothing.
Resident Evil: Operation Raccoon City also had a ranking system, and I honestly can not remember anything about it and rather if you got anything if you got a specific rank other than extra experience.
As for a reward for not killing any zombies or infected in an RE game, there is none. I may have also missed a few games that do have a ranking system and gives you rewards depending on your rank, but these are the ones that I can think of at the moment and I didn't count the rewards you get for reaching a certain rank in any of the mini-games in any of the games.
