Are these 120v DC motors? you could probabally get away with that VA as allowing for average duty cycle, you will probabally never pull 4 amps on each motor at once, the 20 amps is the peak rating of the motor which you should never see if the amplifiers are capable of current limiting to 4 amps. You will need around a 85vac secondary to give you the 120vdc. The Capacitor size will depend on what your required DC voltage will be.
You can use a 12,000µfd capacitor to give you 2.5% ripple at 12a 120vdc.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.