BepInEx patcher v4.0
Installation (from scratch):
https://github.com/BepInEx/BepInEx/wiki/Installation
Installation (from BepInEx v3.X)
https://github.com/BepInEx/BepInEx/wiki/Migration
Changelog:
- Patchless installation
- Mono.Cecil in-memory assembly patching
- Fuzzy assembly resolve (ignores assembly versions when resolving)
- New logging implementation
- Many (many) more small things
If you are not a developer, you are most likely interested in only the first option. Since the installation is patchless, no game files are modified, so it's extremely easy to uninstall the patcher or update your game.
If you are a developer, then the second one is very interesting. Harmony is no longer the only hooking/patching framework, however it is still the preferred. Mono.Cecil is extremely powerful, but due to how unfriendly it can be with multiple people editing the same thing with it in seperate plugins, only use it if you can't do what you want with Harmony.
https://github.com/BepInEx/BepInEx/wiki/Writing-preloader-patchers
Many thanks to denikson and essu for helping me out with this, it would not be possible without their help