I'll assume you have the console connected directly to the repeater; it can be 2-wire, in which case what comes in the repeater's RX, goes down the wire to the console. If the console transmits, even while a user is still talking into the repeater, audio goes from the console to the repeater and out over the air, so the other users can hear. DEPENDING on how the repeater itself is set up, the audio from the RX goes out over the air (straight repeater) or the audio from the console gets priority, and goes out over the air, to other users. This also cuts off any audio coming thru the repeater's RX, and prevents it from going down the line to the console. Mixing the RX and TX audio on a single pair would make a difficult situation; since the repeater already is taking RX audio and routing it the the TX, via one path, if you also add a path where it goes into the 2wire telephone line, it could/would also couple around to the TX audio, resulting in very poor audio, unless a LOT of audio engineering is done to prevent that.
If you set up for 4 wire, 1 pair of wires will carry repeater's RX audio to the console, and the other pair of wires will carry audio from the console out to the be TX'ed over the air. This way, the Console, (if it's setup appropriately) can listen to what's coming in the RX , as well as being able to talk out the TX; very effective with a headset system at the console.