Concrete Floor for 400 Gallon Water Tank
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.