Rabies vaccine should be given as late as possible to a puppy and separated out between the others, so I really think the risk you are taking(by NOT vaccinating yet) is very low in comparison to giving that injection while Bane is still young. As long as you aren't romping around area's where rabid animals are, there is little threat of your pup getting rabies.
He's going to be big if he's already 50# at four months. Keep that growth slow for a sport dog. Bigger is not better!
What club will you be training with?