Is the PLC able to control which laser sensor to activate so that the laser sensor will then start measurement and send data to the PLC? If so, then you may be able to multiplex the RS232 to the 4 laser sensor using relays. All you need is to multiplex the RS232 transmit signals from the laser sensors to that of the PLC's receive signal pin.
On the other hand, if more than one laser sensors could send out data to the PLC simultaneously, then you need 4 independent serial port so that no serial data will be lost. Since each T100MD888+ has only two serial port, you can only interface to two laser sensors (one convert using Auto485) per PLC, and you need two PLCs to interface to four sensors. The two PLCs can handshake with each other using digital I/O line to communicate the pass/fail status of the two sensors under their charge so that there is only one pass/fail indicator light to report a trouble.