Soddy - your situation is very similar to ours, except that we have two men & two women. Whoever brings in a song gets to arrange it the way they want, with varying voice combos, or sometimes just one person singing lead.

We play a number of "heritage" venues without a sound set-up. Lately, however, we've needed a sound set-up, and luckily have access to a fairly decent system - bass and one guitar direct, banjo & other guitar miked, as are the voices - using SM58's & SM57's. We'd like to move to a single mike set-up, but haven't had time to experiment.