How to select speed before set run direction...

Hello, yes, this is my first attemp...

I have built a small car with 4 motors. It is a kind of motorized dolly to record videos and photographs. Which I control by bluetooth.
Now I need to be able to define three or four motor speeds from the same app by sending commands to the arduino. In the application I have several programmable buttons that I can use to send a command character.

Thanks in advance for your help