First, you need to know the formula for volume of a cylinder which is pi times the square of the radius times the height of the column.
Then calculate the volume of the cup.
Then calculate the amount that the level of the pot went down by plugging in the volume of the cup and the radius of the pot.
So. . . cup volume = pi x 1.5^2 x 4 = 28.27 cubic inches
Now for the pot: 28.27 = pi x 4^2 x height lost
28.27/16pi = height
height = 0.56 inches. This makes sense because a cylinder with a larger radius will have a smaller height compared to an equal volume in a cylinder with a smaller radius. You can try this yourself using drinking glasses with different radii.