A 4-pocket jersey adds a fourth pocket—usually zippered and smaller—to the classic three-pocket layout. That extra pocket is ideal for securing valuables like your phone, keys, or wallet so they don't bounce out on rough roads or during a hard effort.
Most riders do fine with the standard three pockets for food, tools, and spare tubes. But if you regularly carry items you can't afford to lose, or you're doing long rides and bikepacking where organization matters, that fourth pocket is genuinely useful.
The only real downside is a slight weight penalty and potentially a snugger fit when all pockets are loaded. If you're a road rider who keeps it minimal, stick with three. If you're touring, bikepacking, or just want peace of mind for your phone, go for four.