When both persons speaking are visible I memorize the lines while the actors practice (having a script at hand might be helpful) and pan the mic between both. If the scene is split into shot/ reverse shot, I also try to get both but my priority lies on the person on-screen. Especialy when the boom operator isn't experienced, too much panning might overburden ...
Do you memorize the dialogue and aim the microphone back and forth between the two people when their lines come up?
This allow you to anticipate one's sentences ending and be on place for beginning of next character's sentence.
Notice that it's important that your microphone is aimed at the person speaking. Which means your microphone can be between ...
I'm all for highest quality, as well. However, this presumably si not Hollywood level production, neither is it a concert recording in static setup.
In location recording, what matters is getting the job done.
In practice, this means dedicated recorder.
How are these 4 channels fed to you? Boom and 3 wireless mics?
Anyway, just a rent a 4-channel solid ...