Hello cpirrmann, welcome to the OptoForums!
To answer your specific question, the LCM4s don't know anything about OPC. The Opto OPC server(s) handle all that OPC communication, then communicate to LCM4s and PACs in a similar way that OptoControl and PAC Control communicate to them.
However, in the PAC firmware, we made significant improvements to how fast that communication occurs, so I'd highly recommend moving at least the more time-critical info from LCM4s to PACs.
In the meantime, your comment about the sessions is interesting. You might do a Wireshark sniff and send the saved data to our product support team to ponder. I'd suspect that the session between the OPC Server and the LCM4 was still open. I'd want to check to make sure that the data appearing there was real/recent if you couldn't connect to the LCM4 via another method. Why you couldn't telnet, etc. is another question.
Do you know how you got into that state? Did a reboot of the LCM4 fix it? (Maybe just disconnecting and re-connecting the Ethernet cable, if you don't want to reboot?)
FYI, there is this "performance" tech note you should check out. It references PACs but the concepts are similar.
Also see this page on migrating your hardware to SNAP PACs (there are options for doing a little at a time, if needed).
Hope that helps!