The first drop is pumped 0 ft.

The last drop is pumped 4 ft.

Okay, let's define 't' as the distance from the top of the tank. This gives us a nice vertical distance function, d(t) = t

The first drop has enough friends to cover

The last drop has enough friends to cover

No vertical relationships are allowed in this inverted cone!

We need a radius function, r(t), such that r(0) = 4 and r(4) = 2. It's linear, so we're done, r(t) = 4 - t/2

Thus,

You just have to think it through. It isn't magic.