It's a folder. There are ways to group track controls and items, but that's a completely different thing in Reaper.
...by which you mean the parent track of the folder.
I think you mean the number channels on the track, but I don't think you need to do this here. You need four channels on the band track.
You're still saying "tracks" when you mean "channels". You would set up the send from the vocal folder parent track to send channels 1&2 to channels 3&4 on the band folder parent.
K. The vocal folder has nothing on channels 3&4, it's sending 1&2 to the band folder's 3&4. By default, that's where ReaComp looks when you choose Aux L&R, but you can actually change that in the pin connectors to any channel or channels that you want.
I'm sure you can get it to work without really thinking about it or knowing exactly the right terms, but if you're trying to communicate with others, it does not help to confuse some of these.
I did mention above that the pre-comp (Yes, I think I called it "lookahead". Whatever you call it.
) shifts the entire envelope earlier in time. It does start to attack early, but it also starts to release early.