In simple words,bus bar differential relay calculates the differences between current entering to bus-bar and current leaving bus bar. There are two kinds of Bus-Bar differential relays 1) High Impedance 2) Low Impedance In high impedance ,usually a stabilizing resistor is connected in series with the coil of differential relay.This compensate for any mis-match between connected C.Ts, difference in their magnetizing curves,Different winding resistance etc etc.Usually the C.T selected turn ratio of all C.Ts feeding differential relay should be same for high impedance relay which is not necessary for low impedance differential. To your second query,the C.T of all our goings can be added at single place such that red is added to red ,yellow to yellow and so on.Hence four wire for all outgoings can be taken in to differential relay.