The formula for the circumference of a circle is $\displaystyle{C}=\pi{d}\usepackage{cmbright}\Huge\$ . The diameter of the circle is 4 centimeters, so $\displaystyle{d}={4}\usepackage{cmbright}\Huge\$ .

$\displaystyle{C}=\pi\cdot{4}={4}\pi\usepackage{cmbright}\Huge\$ centimeters