Caleb, a lift may affect aerodynamics, which negatively affects fuel economy. Remember, those 2nd gen 4runners are slow and thirsty to begin with. I would also be worried about that owner taking it off-road and abusing it...since he has lifted his truck. Hopefully, the owner is just a poser, which is good news for you! I would also testdrive it to make sure that handling is still OK with such a high lift.
One more thing...a high lift (4") can have deleterious effects on suspension and driveshaft components. The wear and tear on such a lift is higher than stock vehicles, esp. if this is a cheap lift or not well thought out.