Thought I should post an update on this - I decided to stick with my current power supply, and try things out. I haven't done any major work with my machine yet, but the current PSU does seem to be handling the load of 4 motors and the fan.
However, I'm writing my own software to control the machine, and tend to run only 2 motors at any given time, just because of the way I'm doing things internally. I am able to run all 4 at once, but I've only tried for very short periods.
Your mileage for this PSU and 4 motors may vary, but things look positive so far.