Be a threat with your secondary trailers. The ability of your post players to score from both a slash and post up cut, or shoot the 3 point shot, opens considerably the opportunities to score on the fast break. Pass ahead to keep your numbers advantage and reduce the total number of defenders. A 3 on 2 produces easier scores than a 4 on 3, just as a 2 on 1 produces more scores than a 3 on 2. Any dribbler who can hold a defender by his dribble that covers ground, can pass head to create this type of advantage.
Most fast breaks are two slow because players dribble too much.
This requires a great deal of sprint work conditioning, and belief by your players, but when you can achieve these goals, you will really have a fast break. For younger kids add a second to each mark or adjust for their athletic ability. In your case, if your 2 man is your best rebounder, teach your 4 man to sprint, and use the 2 as a dangerous 3 point shooter on the trail, or even to post them as the secondary trailer, before initiating your offense. This is a little unusual but changing roles to fit your teams personnel is the most effective way of letting your players play to their strengths without changing your system, just change the numbering. Thanks for Asking the Coach.