Interfacing an RS422 driver and receiver
You will need to look at the maximum voltage that the inputs of the receiver can withstand. In a differential receiver it is normal to see a maximum voltage spec for the common mode voltage range that the receiver can withstand. The maximum voltage of the outputs of the driver plus the maximum voltage difference between the GND at the driver and the GND at the receiver needs to be within the common mode voltage range of the receiver. 

It is typical for the receiver to specify the minimum differential sensitivity voltage level at which it will reliably produce the correct 0 or 1 output depending upon the differential input voltage polarity. This may be the +/-400mV rating that you noted for the part you looked at. It is possible to find parts sensitive down to +/-250mV.

The receiver responds to a much lower level of differential swing than that produced by the driver so that it will still detect the signal properly even though it has been attenuated a huge amount after being sent down extremely long cables. The termination resistors used on the signal lines to prevent reflections and ringing of the signal also contribute to the overall signal attenuation from driver to receiver.