Yes, it is possible to synchronize more than 3 DL750 ScopeCorders by using the trigger signal, in a one-to-one connection method, as long as the limits of the delay time is not exceeded.
For time sensitive applications, it is possible to connect the instruments as shown below. However, please shorten the wiring length between the DL750's as short as possible (for example, using T-adaptor).
Yokogawa data acquisition systems give you the most flexibility and power to measure, display, store, and even actuate any number of physical or electrical phenomena.