What should I do if the ball goes for a four or a six?

Boundaries are signalled by the main umpire, not by you. A four is a hand waved side to side across the chest; a six is both arms raised straight above the head.

If the ball reaches the boundary near you and the main umpire may not have seen it clearly, such as whether it bounced first or a fielder touched the rope while holding it, tell them what you saw.