2 wire com = party line com that loops from beltpack to beltpack & carries all audio smooshed together. It is an unbalanced audio signal with Clearcom(tm).

4 wire com = Party line com separated as input & output audio signals. Could be on balanced XLR, RJ45 or other connector. Each com channel would have separate input & output signal. Unless they are hooking com up to something else, most users don't deal with 4 wire com.

I heard "3 wire com" today! That's a new one. All the major com providers use 3 pin xlr mic cable to connect beltpacks, but that's not really a term.