I agree with rdrast. Let the datalogger keep track of time.
If you choose not to let the datalogger handle timestamping, then pick one of the PLCs as the master. Wire an output from that PLC to an input on each of the other three. At some predetermined time (once a day, once an hour, you decide) as determined by the master PLC's RTC, have the master PLC turn on the output. When the other three PLCs see the corresponding input, they set their clocks to match the master's clock.
For example, every hour at 15 minutes and 15 seconds past the hour, the master PLC briefly turns on the output. The other PLCs see the signal and set their clock's seconds value to 15.