I'm not sure about at 70, but you can solo instances on the way there with just 3. I'm guessing you'll need at least 4 for the level 70 instances, but even then you might run into some problems.

Last I checked hunters still have a deadzone, it is just 1/3 the size(5-8 yards to 3-4 yards).

You can heal and damage at the same time with shamans, have one heal and three damage.