Have done all 4mans up to Tatooine, mostly with 1 or 2 other people(so not full groups) and rarely dedicated tanks, worked np. I'm a healer though, and that helps a lot. Can duo them easily and can solo SOME of them too as a commando.
Hardest group quest yet has been a 2man heroic, on Taris with crazy OP rakghouls, they were tearing people apart and I had a hard time healing, even though it was half silver half elite packs and stuff. Ridiculously hard for a 2man.