your motors are rated at an average of 3A per phase , with bipolar windings ( 2 phases ) that means each motor requires 3A x 2 , so thats an average of 6A per motor
so 4 motors @ 6A each , have a maximum rating together of 24A , if they happen to be at a worse case situation of requiring maximum current on all motors ( ok it's not going to happen often , but you need to be able to handle this ) .
so your options are a single power supply that can supply 24 A or split your motor supplys between more than one power supply , but allowing each supply to cover the current needed for what ever it is connected to i.e 6A for each motor in your case .
in this case current capacity is more important than voltage , as the controller supplying the motor can be current limited by onboard resistors on a per motor basis , and therfore the voltage is also limited to .
your voltage requirement needs to be as i said before , as a crude guide 5 times the rating but it's not critical so 24v - 30v will be fine
just keep it below the maximum requirement of your controller by say 5v
as i said this is a quick crude way of finding power supply ratings, without going into all the mathmatics and formulas , so yes everyone will find their own disagreements
if they wish to look for them , however it works in oractice.
i am just trying to keep it simple for the electronicly challenged amongst the community .
Dave