Your drivers are independant and the controller can drive all steppers simulataneously.
Most drivers, unless full stepping can drive both coils at the same time. So worst case you could draw 12A.
However, chances are unless you are driving a very heavy or stiff load, the current will be drawn in small spikes which will average out a lot less so you may just get away with 4A.
BTW old PC power supplies usually can do 12V @ 8A or more and are pretty easy to find.
On the other hand, you would probably want to go to a higher voltage anyway, to get better performance from the steppers.