I have a quad. I impulse bought two bucks from a local pet shop, had them neutered, they fell out big time. I separated them for ages. In the meantime I got two does from a rescue.
Decided to bond them all together. It went well. They have moved house (well, shed in their case) with me twice since then and are totally fine.
http://forums.rabbitrehome.org.uk/s...now-a-quad-More-pics!&highlight=bonding+begun
I would always have a litter tray per rabbit and at least one other so that they can all have a territory. I would also keep the dirty litter trays when I put them into their home to be so that they dont have to reclaim them all over again.
I have three 32 litre under bed storage boxes for them which I change each week. They have free range time in the garden for about two hours a day. Having four has been no more worry than having two. The more set ups the more hassle. Its great having them all together in one shed/run.