Bricks, Masonry, Asphalt and Concrete - How thick a slab?

I have 2 large plastic tanks that hold 250 gallons each. Water being about 8 pounds per gallon, the two tanks should weigh in around 4000 pounds when full.

I assume that I'll need to pour a small slab (3'x6') to hold the two tanks.

http://www.mrdove.net/images/Picture%20130.jpg
tried to link my picture but it didn't work. Can you not link or post pictures here?

What precautions do I need to take to make sure that the slab will be secure under so much weight? What thickness of concrete? What material under the concrete? Anything mixed in with the concrete?

Also, would these tanks sit safely in the dirt if I were to try that route? Would the weight and the fact that they are plastic cause problems? Would it help to fill them slowly so that the ground can compact under less than full weight?

A slab formed with 2x4s (3-1/2" thick) would be adequate.

A driveway slab can support more the 4000# at the 4 small areas the tires sit on. You are spreading the 4000# out evenly over a much larger area.

Dick