I'm back!
Went with the following setup in the end:
As you can see, it's vocalist dead center, drums and bass slightly behind to her left and right respectively. The guitars are pushed to the extreme ends for stereo effect.
It was one PA speaker firing dead center outwards, with a second lying down firing upwards back at the vocalist as a monitor. All four mics were fed through the PA, and nothing else was in the speakers, given the very lacking equipment available.
For 'monitoring', we had to hear each other, and surprisingly it worked most of the time. We had two short sets this evening, and worked to set up well before the first set and to correct the mix after the first set, from the audience's feedback. Surprisingly, everyone heard everyone else most of the time, except times when the guitars got too busy and the guitarists couldn't hear each other. I got them to anchor to the drums before the gig, so it worked out well.
The equipment was adequate for the venue and crowd, I think. There were no issues of audibility, contrary to what some of you were worried about.
The problem was a boomy wooden stage that I forgot to factor in. I turned my bass down on the Genz Benz Shuttle 3.0 and went for a grindy high mids heavy tone to compensate and cut through instead. Conveniently there were cheerleading mats (those foamy squares with interlocking edges that cheerleaders practise on) backstage, so we 'stole' one to isolate the drum and minimize boomy-ness.
One more problem was the hall itself. Glass walls/windows on three sides and a concrete wall on the last. The sound was variously described as too bassy and too screechy by various people, depending on where they stood. I could only shrug, and hope for better locations in the future.
Anyway, thanks for all your advices, fellas. Much appreciated. I'm sure I'll need your help again in the future. But for now, at least it's one gig done.