A man runsa distance of \(9 \mathrm{~km} / \mathrm{h}\) for the first \(4 \mathrm{~km}\) and then \(2 \mathrm{~km} / \mathrm{h}\) for the rest of the distance. The whole fun takes him one hour. His average speed for the first \(4 \mathrm{~km}\) is