I have a team of 4 hunters and a priest in the mid 60s, and they are definitely fun to quest on. As for PvP, I agree with the others that they are best suited to BG like AV or maybe AB.

As for arena, the main problem I see is that you won't be able to take advantage of all your abilities and thus lose out on some of the tools that make hunters viable in arena when played solo. Disengage and deterence are great, but how do you handle using it with 4 characters? You'll end up with some interesting facing and follow issues no matter how you handle it. Pets can be an annoyance, but they die pretty fast with minimal focus fire. Hunters themselves are also pretty squishy if casters manage to get a bead on them. There is some potential here, but I think it's going to be tricky to iron out the fine details. If you pull it off, I'd love to see it in action.

- Souca -