Depends on how much trend has been worn down on the others. Sometimes the tire shop can shave the new tire down to match the remaining tread of the other 3.
It's generally the rule of thumb to have the same tires on all 4 corners. Even more so on all wheel drive vehicles. Different tread patterns and different compounds it tires can effect the drivability of the car and the longevity of drivetrain.
I think a lot of shops are doing that now. My wife's Audi A4 Quattro had a blow out and we got 4 new tires from Americas tire because the one tire that blew was unrepairable and the other tires we got a pro rated credit on them. I think we paid about $200 for a set of four mounted and balanced.
