The radius can be viewed as a function of the height. The radius of the top of the water will be proportional to the maximum radius of the cone to the maximun height of the cone: 4 inches to 16 inches. So the radius is always 1/4 of the height of the water.
(r2h)π/3 / = ((h/4)2)hπ/3 = πr3/48
V(h) = πr3/48.