Good question if only because it approximates. I imagine you might have been told to do the question in your head.
Area = pi r^2
r = d/2
r = 4/2
r = 2 feet
pi = 3
Area = 3 * 2 * 2 = 12 square feet <<<<<< Answer.
Somewhere in 1 Kings Ch 7, the Bible makes reference to pi being 3. It's a pretty good estimate considering the number system the Bible used.