If you're using it for a battery charger to charge at 5amps, 4.2 wouldn't be enough. It might charge at 4amps, then again, PC part makers overrate their stuff, so it might not. It certainly doesn't seem worth the hassle to use a PC supply if it only puts out 4amps.
If you are desperately cheap, I have a very small power supply that outputs just 4amps you can have for free