What kind of bike, where's it been ridden, how's it been stored? Is there really water, sand in dirt in there or is it just a possibility the mechanic mentioned?

Four years old, it's probably some sort of cartridge bearing. If it's ridden on streets and stored inside, the BB's probably fine. If it's been regularly ridden through mudholes and stream crossings, then washed with a high pressure spray, it may be shot.

The best approach for a bottom bracket is to replace it when it really is shot. Test by trying to wiggle the cranks perpendicular to the plane of the bike frame. Even when the BB does break, you can ride it until you can get it to a bike shop to replace it. It'll sound like a Doritos commercial, but you'll still be able to pedal the bike.