You could probably get away with 4 hunters and a healer. I leveled a 4 hunter group pre-cataclysm, and granted I had dwarven handcannon heirlooms, it was pretty simple. The pets were expendable throwaway tanks. You had to be extremely reckless with your pull, to get all your pets killed. Typically you could easily res pets that died on a rotation, all the while just blowing everything away with your hunter DPS. With threat ping ponging between your pets, they rarely died anyways, when they had hunter heals+healer healing them. It's a group I've considered for classic.

Would it work better than having a dedicated tank? Yes and no. Yes in that, you're gonna clear a lot of instances a lot faster having 4 pure DPS, no in that, there might be some boss fights/pulls in some of the higher level dungeons where having a real tank would be better.