The traditional 4th class is the divine class whose magic comes from a different place than the mages magic.
I don't think that class can exist in Thedas and not mess up the entire gist of the world.
So, I am fine with 3 classes, but I would like a lot more depth put into the mentioned favorites, like mages with inclination towards warrior or rogue physical abilities, who use magic to augment their abilities, rather than become the generic robe wearing book worm, frail, pasty dude.
Is it not possible that a mage who eluded people for years might have did it via stealth and the more rogue like skills?
Is it not possible that a mage that would succesfully defeat or avoid the templars, might have did so by becoming a decent warrior who augmented his physical abilities with magic, rather than counting on his magic to affect the very resistant templars?
I'm just rambling, but I find it more interesting to have more focused forms of magic than just the robe wearing dudes who throw fireballs.