I did almost all normal BC instances with this composition and it worked just fine. I would run 3 felguards and an imp, pulling out voidwalkers for bosses if needed (most of the time it wasn't). Once wotlk came out I added a tank.

In my opinion I'd guess that 4 warlocks + priest would be able to do all normal wotlk instances just fine, but heroic content (just like in BC) would be out of reach.

It is incredibly nice to have 4 dpsers in a group -- it makes things so much easier. This is what makes 4 shaman + tank groups work so well (in addition to bloodlust).