Fiddler ?

I know that they need brackish waters, and I have set up everything (low water level with things for them to climb on to get out of the water, filter, light, I know about the food) the only problem I am having is that I am unsure of what type of salt to use, and how much to put in there. I have about 4 gallons of water in a 30 gallon tank (so they have plenty of space to swim and be out of the water), and I have read several places not to use aquarium salt. So basically, what type of salt, and how much salt do I add to 4 gallons of water for them to stay healthy and live long?