Unless there's one (or a couple) kids with whom you're particularly close, I think it's an all or nothing deal. You can either find something for ALL the kids to do or have no kids in the wedding party at all, so as to avoid playing favorites. I think most parents would understand that.
If it was my wedding and I was close with that many kids, I'd try to involve them all. Just off the top of my head, here's a few options:
1) Have them all be 'flower kids.' This would be really cute, but you run the risk of little boys not wanting to do the same job as the girls and the whole cootie factor. If you think the kids can handle it, this would be really cute.
2) Some combination of flower girls and 'bubble kids' or whatever you're throwing. I'd stay away from appointing one boy as ring bearer. Obviously, four boys cannot all carry one ring unless they played relay down the aisle, which I wouldn't suggest. I'd nix the ring bearer and put the boys in charge of rice or something.
3) If you're having an aisle runner, why not have the little boys roll it out before you enter? So you'd have 'Aisle Runner Boys' and then the flower girls.
4) Could they hand out programs, bells, fans, whatever you're giving to people? Or do the parents all want them 'in' the wedding?
5) Could they announce your arrival somehow? Like walk down the aisle playing some sort of instrument? Triangles, perhaps? Or ringing bells? That might be cute.
I can keep going, if you want to hear more, but they may soon cross the line into 'ArtBride's Crazy Schemes' territory. I think you should be able to come up with a way of including all the children, but you'll have to get creative and think beyond the traditional flower girl and ring bearer roles.
