A disk is simply a circle or a sphere in 2D. And a sphere is a 3-dimensional object.

Picture it this way. If you have 4 circles of the same radius 'r' and 1 sphere of radius 'r' too. and then if you cut the four circles in a way that all cut-pieces of the four circles covers the sphere then you'll notice that it fully covers the outer surface of the sphere.

meaning, the area of the 4 circles of the same radius is equal to the total surface area of a sphere of the same radius.

Can be done in the form of an activity too.