if the diameter is 4 units, then its radius is half that, or 2.
[tex]\bf \textit{area of a circle}\\\\ A=\pi r^2~~ \begin{cases} r=radius\\ \cline{1-1} r= 2 \end{cases}\implies A=\pi 2^2\implies A=4\pi \implies \stackrel{\pi =3.14~\hfill }{A\approx 12.57}[/tex]
