Shielded twisted pair balanced transmission line implementation
Looking into the source, it is balanced regarding impedances.

Looking into the receiver it is not at all balanced because you have the differential amplifier's output (close to zero ohms impedance) connected directly to the upper wire and the 1k2 resistor is doing nothing. A balanced receiver looks like this: -



To add a little more, the source is impedance balanced but the impedances are high. In effect each signal wire has an impedance of 10k/4 + 120 ohms = 2.62 kohm and this would degrade the signal. You need to provide a buffer that then uses the 120 ohm resistors in each differential output like this: -



Above picture from here.

So, the signal entering the above diagram at the left can be regarded as the summed value of your four voltage sources (V3-V6). Summing four differential sources as you have shown is not very practical. Use a summing amplifier like this: -