How do you time-synchronize the 4 RPi? When you power them off, you lose the sync. So it is difficult to do it correctly.
Also it may be possible that the different RPi have different clock drift over time? i.e. 1.0000 hour on one of them = 1 hour + 20 milliseconds on another one
https://github.com/hcfman/sbts-aru
uses a GPS to synchronise the time with and then even when running completely disconnected from any network the clocks will be accurate to real time with less than 1 microsecond of error. Typically the system time hovers around 100 ns or less from the real time. And I’ve tested this by triggering interrupts on gpio’s on two devices with the same switch and printing the time.
Syncing to sub microsecond accuracy is usually achieved in under a minute.