Part 1 additions to Leadfingers and Richard's comments...

Your current speaker situation is two FOH speakers and two monitors, with the main channel driving the two FOH speakers and the monitor channel driving the monitors. When you've got one amp channel driving multiple 8-ohm speakers, the impedance that amp channel sees is (8 / number of speakers). So both channels already have 4-ohm loads on them, and any extra speakers will overload your mixer. The mixer will be unhappy about that and may express its displeasure thermally, if you catch my drift. ;-)

So why the extra output socket? Most speakers have two connectors, letting you chain speakers together (technically the phrase is "connect in parallel") with one cable from the amp to the first speaker, and a second cable from the first speaker to the second speaker. Some don't though (particularly old ones or cheap ones), in which case it's handy if the amp gives you an extra output socket. It doesn't give you carte blanche to jack another pair of speakers in there, sadly.

Going into Leadfingers' comment on coverage in a little more detail... Speakers project sound over a distinct cone-shaped area where they're pointing. In a pub/room/hall, this will bounce off walls and might even out enough not to be an issue. In the open air, that ain't the case. Say you point the speakers out front to get the sound to the back of the area - and it does, so that's great. Then ironically, anyone near the stage can't hear a thing now, because they're out of the speakers' coverage. Or if you turn the main speakers inwards, now the people at the back can't hear. Two pairs of FOH speakers keeps every happy. But as I said, your mixer won't let you do that, because four speakers on the FOH channel leaves that channel pushing 2 ohms, and the mixer won't do that and live (not at any significant load, anyway).

Re point 3 - the mixer will be fine at full whack. But be warned that this might reduce your headroom, and large peaks could cause the mixer to distort, which is very unhealthy for speakers. See Richard's comments.

Re point 4 - yes, these are two separate channels so you can do what you like.

As for generally whether it's going to be loud enough, it depends entirely on the music and the event. If it's death metal, send Igor out for a bigger lighting conductor. If it's general folk stuff, you should be OK.

Part 2 additions to Richard's comments...

The one thing you *really* need to do with EQ is stop feedback. This means that on the monitor channel you kill any frequencies you don't need. 5K upwards and 160 downwards, you can lose the lot. High frequencies are where most feedback happens, but it doesn't hurt to get rid of the low stuff too.

For FOH, it's a case-by-case thing. You can optimise the EQ for one singer's vocals, but that won't help when you've got two guitars and two other vocalists also going through FOH. Relatively flat is safe. Although as Richard says, you could lose the low-end without anyone noticing. The main thing is just to use it to minimise feedback if you can - you *really* don't want that!

One tip though. If anyone is definitely backing vocals only, it's a good idea to cut their treble a little. Above about 1-2KHz for men or about 3-4KHz for women, those higher frequencies are telling you not what the note is but what the *consonant* is; below that, it's telling you the pitch of the note they're singing. And it's the consonants that make the words. So if the backing vocalists' trebles are cut a little, you'll hear everyone's notes clearly, but the lead vocalist's consonants will come through above everyone elses, giving the perception of her words cutting through the harmonies. Of course, if everyone is singing lead at some point, you can't do that. Or you can, but you'll need to be constantly tweaking the mixer between songs, and that's a recipe for getting things wrong so best not to go there.

Graham.