Umpire needs to alert the catcher on a 3 ball steal.

When a runner takes off with 3 balls on the batter, there exits a possibility that the batter could walk. However, the catcher can't anticipate this and must be prepared to throw the runner out. This can be a very difficult proposition for the catcher if the umpire isn't an assertive guy behind the plate.

When the ball is received for ball 4, an assertive umpire will YELL ball 4 so the catcher knows he doesn't have to make the throw.

This is one of those plays where nothing could can come from the catcher throwing the ball. If he throws the runner out he was entitled to the base anyway.

If he throws the ball away, the runner can advance to 3rd. Absolutely a miserable proposition for throwing the ball.

The umpire must alert the catcher early. This is one of those situations where managers could and would go ballistic if the umpire isn't aware of this situation.