A 5 amp power supply would be the minimum requirement.

Most well made power supplies are bulky because they use giant transformers and capacitors. I use one like this:


I actually use a 25 amp because I have several things that it powers. If you have any plans on charging at 4amps, then I would reccomend something a bit larger than 5 amps though. The one in the picture is a 10amp and costs about $50 with shipping in the USA.

The relationship between volts and amperage is defined in Ohm's law I=V/r but you don't need to concern yourself with that where chargers and power supplies are concerned. You just have to know what your power requirements are. For your charger to be able to charge a battery at 4 amps, you have to have at least that much plus whatever is necessary to operate the charger. 5 might do it, but you want to play it safe and have some extra juice. 6-10 amps for a single charger is fine. If there is the possibility that somewhere down the road you want to operate two chargers at the same time then double the amperage, or get 12-20 amps.