Any power supply that you're going to run across will supply a fixed voltage, and will be able to deliver up to the rated current.

Your charger needs some voltage (11-18), and will suck as much current as it needs, up to 4A.

So as long as the power supply you get can deliver 4A, or 5A, or 2000A, you're fine (well, except the 2000A supply might be a tad big).

Life's a bit more complicated if you're looking at PC power supplies -- they have to be modified a bit to deliver just 12V out.  Is that where you're going, or are you looking for purpose-built 12V out supplies?

Paul, I think you have the wrong simile.  Geese have excellent navigation systems and seldom get lost.  A person is sometimes said to "feel real loose like a long-neck goose", but that describes a different phenomenon.  Hope this helps.

Thats what mine is, a simple compy PSU, heavily modified.

Mine is this: http://www.ple.com.au/ViewItem.aspx?InventoryItemID=612038&CategoryID=519 and at $114 AUD it's pretty cheap, and with Heavy 12v bus bars it's pretty much set up for what I want to do.

Just buy one of these on eBay.
I use one and it works great!!
eBay # 151144464439
He also sells them in pairs for even more grunt.

Paul,
Another option to consider is the Radio Shack, model 13.8VDC 15A Power Supply for $85. I have used this for several years with a Thunder Power TP 610 C charger with excellent results, can power two chargers off it if you like.

You could also consider the Thunder Power TP 610C-ACDC charger that includes the power supply built-in. I have this unit and it makes going to a contest easier as you only have to remember one piece of equipment to bring. It also has the side DC clips to run from 12V battery (if you do use the battery for charging make sure to set the voltage level to 11 V, you can actually leave it there all the time, if you don't change it you'll get a low voltage message and it won't charge).

Best,          DennisT