How do I convert voltage square wave signal to 4-20mA signal
It is really a waste of capabilities of the sensor that you linked to try to convert its quadrature square wave output to a 4-20mA current loop signal. The sensor is really designed to be able to sense both direction and RPM of the gear wheel. RPM of course related directly to the frequency of the square wave output. 

To do the conversion with off the shelf hardware you want to search for "frequency to current converter 4-20mA". You will find many products that will do just what you want.

On the other hand it would make way more sense to get a sensor that directly outputs the 4-20mA signal in the first place. Search for "tachometer with 4 20ma output" to see many things to consider.