[tex]~~~~~~\text{Arc length} = r \theta \\\\\implies 4\pi = r\left( 45 \times \dfrac{\pi}{180} \right)~~~~~~~~;[\theta ~ \text{is in radians.}]\\\\\implies 4 = \dfrac{45r}{180}\\\\\implies 45r = 180 \times 4 = 720\\\\\implies r = \dfrac{720}{45}\\\\\implies r = 16~ \text{units.}\\\\\text{Now,}\\\\\text{Area} = \pi r^2\\\\~~~~~~~=\pi \cdot 16^2\\\\~~~~~~~=256\pi~ \text{units}^2[/tex]
