Cool idea, having a 4th bird would be fun, though it does bring up a few issues.
1: A B-17 can stay up for, well, ever. Whats to stop someone from doing a spiral climb above their home base, then when they get to altitude, now they have a 4th bird. Now they can attack any base they want to, not just strats.
2: Say you're on a strat run, you have you're 4th bird, you get to target and drop. Remember the drones drop at the same time you do. When you head home the damage that you've done would already be regenerating no matter how many birds you brought along. In my opinion, the reason you go after a strat is to make an impact on the country you're attacking versus grabbing damage points. It's probably better to tweek the times.