This is a tough one on which I should recommend.
For RE4: The game is mega customizable as you can switch up and make new things withoout having a sh*t load of glitches with mods. You can play with any console base controller as long as the controller as a USB end to it. Fun with and without mods. And if I am not mistaking, it should be easier to install. You can use mods to play as other characters from the series.
For RE5: Amazing graphics. Fun co-op. Compatible with some mods. Can be played with a console base controller as long as the controller as a USB end. Fun with and without mods. So d*nm fun. Has a mod where you can take out a co-op partner. The only thing that RE4 does have that RE5 can't not have are mods that can actually let you play as every single character from the RE series including Rebecca, Chris from RE Remake, CV, and 5, Jill, Wesker, Claire, Carlos, and all those good characters, even Nemesis gets a playable mod, as long as you can find the mods for them, then you can play as them. RE4 can also have mods that let you play as other Video game characters like Snake, Lara Croft, Heather Mason, Agent 47, and more. The nearest to RE5 of playing past characters are Claire Code Veronica costumes for Jill and Sheva even though they only put the ladies in costums rather than actually makeing them into the real girls.
Against RE4: No co-op. Lags alot. Sh***y graphics at some points. Glitches alot with some mods. Babysit Ashley most of the time.
Against RE5: Some mods are very glitchy and hardly useable. Can't be compatible with some mods. The AI likes to abuse some of the healing items.
So far I think that RE5 probably is better on the PC than RE4. RE4 on PC is almost like RE5 on PC except the absense of co-op and that on RE5 you don't have to help Sheva alot, but Ashley is kinda of a pain to help as she dosen't like to move at some points, 100% can not help you at all, and she dies easily, as with RE5 AI they can help you and are obident, but will sometimes do some stupid stuff about once or twice on each level.