For 4 separate instruments, you would need to send a trigger signal to all of them to initiate a sweep. With GPIB, you have the GET (Group Execute Trigger) or you can use an external trigger (i.e. a voltage signal). I'm not sure how the software trigger is implemented with USBTMC instruments. That's a technical question for the scope vendor.
p.s. In the future, post similar questions to the
Instrument Control board and not the multifunction DAQ board.