My typical answer to these questions is talk to a Licensed Engineer because there are a lot of considerations that are needed. I firmly stand behind that.
On this project make sure you're using the correct information.
Water ~3,300 pounds
Tank ~ 180 pounds,
Concrete (4in thick without reinforcement) ~2,400 pounds
Piping ? other accessories?
Thats only part of the weight that the ground below your tank will support; how sure are you that the ground will hold that? Typically those tanks don't overflow at their rated capacity, they have 5-15% excess capacity, this needs to be designed to passively let the excess overflow or designed for the tank to be filled the overflow level.