Haven't tested the workaround yet, but theoretically you can bypass that dialog by patching EoCApp.exe:
With a hex editor, search for "41 4A 00 40 0F B6 C7 48 8B 4C 24 60 48 33 CC E8 34 38 AA FF".
Replace the bytes "40 0F B6 C7" with "31 C0 F7 D0".
I just made a copy of the exe and hex edited the values. This prevents the error message. Everything appears to be working like the original 4 player mod.
Haven't fully tested everything yet, but it's looking very good.
Connecting an unmodified .exe to a modded one seems to work fine as well, meaning the entire 4 player mod is fully serverside with no additional downloads needed for players.
Anyone doing the mod would have to do it themselves, since distributing a modded .exe is likely against the EULA/TOS.
I was trying to do this with a memory editor like Cheat Engine but not having any luck finding the memory value for this prompt. For my own education, how did you find the memory location for that error?
