Where can I find UE4 API

Hello getting UE started for the first time and I want to write a UE4 plugin, specifically modding towards SCUM (which I understand is UE4 based). where do I find that API package to download? setting up UE editor for the first time, the only choices I have are UE5 based, which I do not want/need at the moment. modding is almost exclusively C++ yes? in what lang versions, runtimes, etc? how much support can we lend outside of “vanilla” C++, for 3P libraries perhaps like BOOST or what not. after that, I assume such paths are included in that environment for things like game, player controllers, so on and so forth? Cheers, thank you…