How can I control 4 separate Motors from a single H-Bridge?
The best solution for this is to use four separate H-bridges. It will be way easier to provide start/stop gating of the digital logic level signals that drive the inputs to the H-bridges than it will ever be to messing with the high power signals going to each motor. 

The four H-bridges has the additional significant advantage of spreading out the high power drive across more circuits rather than stressing one circuit to much higher levels.