4 players, 3 balls

Players 1 and 2 must never stop the ball.
Player 3 calls and passes the ball to player 1.
Player 4 calls and passes the ball to player 2.

1) Players 3 and 4 pass easy to players 1 and 2 and stop the balls.

2) Players 1 and 2 dig the balls hit by players 3 and 4.

3) The passing must be continues.