Lunch with Muhammad

Probability Level 4

A newly-opened restaurant has $5$ menu items. If the first $4$ customers each choose one menu item uniformly at random, the probability that the $4$ th customer orders a previously unordered item is $\frac{a}{b}$ , where $a$ and $b$ are relatively prime positive integers. What is $a+b$ ?

This problem is posed by Muhammad A.