First we need to calculate the volume required to hold 4000 liters of water. 1 cubic foot will hold 28.317 liters of water, so we will need a tank that is 4000/28.317 cubic feet or a tank of approximately 142 cubic feet. Since we are designing the tank lets make it easy to construct by keeping the sides even measurements. A tank 6' X 6' X 4' has a volume of 144 cu. Ft. And would hold 4078 liters of H2O. You can shave a little of each measurement to get to exactly 4000 but that would leave no room for expansion.