#### 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 then find the angular acceleration as:
$\alpha = \frac{\Delta \omega}{t}$
$\alpha = \frac{60\pi~rad/s}{4.0~s}$
$\alpha = 47~rad/s^2$