Yup, and that could be challenging to build 
I assume you’ve discovered the rotary encoder specs for the integrated NEMA17 motors that FarmBot supplied ? ( I think they’re wired for differential A and B “line driver” because of the very long cables )
. . and the “Plug Everything In” page which clearly shows that the X2 motor encoder is not connected and a text block which says
Only one x-axis encoder is currently implemented in the firmware, so
the X2 encoder wires do not need to be plugged in to the RAMPS board at this time.
Yup, agree. Again that’s Custom firmware . . provided that the AtMega MCU can handle the additional CPU / interrupt load when all 4 encoders are signalling at max axis speeds ! (???)