Although they are very similar they have small differences. The most important: RS-422 has only one single Commanding Device (master) and up to 10 Listeners (slave). In most cases RS-422 is used for only one (commanding) device to one other (listening) device. RS-422 is only defined with 4 wires (separate pairs for transmission and reception).
RS-485 can have multiple commanding devices and up to 32 devices listening on the bus. In most cases RS-485 does use only two wires.
2-wire communication is half-duplex while 4-wire communication is full-duplex meaning that with two wires at a given time one device can write to the bus. With 4 wires the controlling device may write while one of the listeners is writing back an answer. As 4 wires usually means RS-422 and that is usually used only for point-to-point connections both communication partners can write at the same time.
I don't know what you mean with "common mode signal wire". If you mean that you need two wires per signal the answer is yes because the signal is transmitted a difference between the two wires and it doesn't have a fixed offset to a common ground. This makes it immune (mostly) to inducted noise signals.
You don't need a ground wire between the communication partners and it's usually not a good idea to have such a wire if the partner not also share the power supply.
@pylon thanks. Thats what I wanted to know. what i meant is I have RS422 connector that has positive, negative, common mode and shield wires. I am sure it will work fine with only 2 wires but I have connected shield wire to ground. Connecting Common mode wire to ground do any good for long distance communication?
As far as I know RS-422 is not defined for 2 wires only for 4 (one differential pair for each direction). I also never heard of a common mode wire to be in a RS-422 cable. What is that wire used for in your setup?
Do you have a link to that RS-422 cable?