The specs for the G540 kind of are dictating your limiting power supply voltage for you as they are rated at 50v DC....
The current rating of your steppers is 3.5A, and its unlikley that they will draw 3.5A x 4 = 14A x 50V = 700 W
So you're looking for a lower supply rated at less than 700W and less than 50V DC allowing a margin for the back EMF from the drives.
http://www.cnccookbook.com/MTLatheCNCDrivers.htmProbably looking at a 44V 600W one of these
http://www.antekinc.com/600w/http://www.pmdx.com/PMDX-137My personal advice would be to not use a switched mode power supply but look for a toroidial transformer type power supply or build your own (in which case you will be looking for a transformer rated at less than 700W and 35V AC. The reason being is they tend to deal with the back emf better and absorb the current flow they are also quieter when you're running your steppers.
There is a bit of info here for you to read up on.
If you're looking for ideas for the rest of the plasma, have a look at this link, it may give you an idea about doing an easy to expand system.
http://www.instructables.com/id/CNC-Plasma-Table/Unfortunately everyone tends to buy the steppers, drives and BOB before they build their machine... and really they should consider doing it the other way around...
Hope some of this helps.
Rob