Answer
$\alpha = 47~rad/s^2$
Work Step by Step
We can convert 1800 rpm to units of rad/s:
$\omega = (1800~rpm)(\frac{1~min}{60~s})(\frac{2\pi~rad}{1~rev})$
$\omega = 60\pi~rad/s$
We can find the angular acceleration;
$\alpha = \frac{\omega}{t}$
$\alpha = \frac{60\pi~rad/s}{4.0~s}$
$\alpha = 47~rad/s^2$
