There are several ways to do things. My suggestion is to just set up two stereo tracks going into the master bus, and set the volume of one track relative to the other in the ultimate mix. Once you have a mix you like, you render it intoone stereo file. You could also create a left and right track for each set of microphones and mix the four tracks instead of two stereo tracks - you could pan channels as well as setting the volume of each of channel relative to the others. For many tracks, you can create other busses and send tracks to busses before they go into the master bus (ir all vocals into a bus). I've only done mixes of two sets of microphones, usually a SBD and on stage microphones. In this case, you also have to time adjust, which I do by nudging on of the stereo tracks relative to the other.
Create stereo tracks by using wave-agent to save tracks from the polyway into a file.
Drag stereo wav to the track srea on the left of the reaper screen
Set volume for tracks using the controls at the bottom - master track, track one and track two in this case
Open render dialog and make sure bitrate is what you want - in this case I recorded at 24 bit and am rendering to 24 bit