How much water pressure would you have from a tank 50 feet high through a 4 inch pipe?

Assuming the tank is full, the 'head', or pressure expressed in feet of water, is 50 feet at the bottom of the tank. Water weighs 62.4 lb/ft^3. At 50 ft, the pressure is 62.4 X 50 = 3120 lb/ft^2. In lb/in^2, or PSI, the pressure would be 3120/144 = 21.7 PSI, since there are 144 square inches in one square foot. The 4 inch pipe size is irrelevant, unless the water from the tank is flowing through the pipe, the pipe is long, and you are interested in the pressure at the end of the pipe. But you would need more information for that, such as the length of the pipe, the details of any elbows or restrictions, etc.