Suppose that the triangle ABC has A = 90^{0}, B = 60^{0}, AB = 4. We construct the rectangle ABCD with center O. By the assumption B = 60^{0} , assune that OAB is a equilateral triangle; OB = AB = 4, BC = 8. By the theorem Pytagor we have

\(AC=\sqrt{BC^2-AB^2}=\sqrt{8^2-4^2}=4\sqrt{3}\). So \(BC=8,CA=4\sqrt{3}\).