6-digit Arrangements

Probability

The digits $1,2,3,4,5,6$ are arranged to form a 6-digit number. The probability that the number formed is divisible by 4 can be expressed as $\frac{a}{b}$ where $a$ and $b$ are coprime positive integers. What is the value of $a + b$ ?

Details and assumptions

Each digit is only used once in forming the 6-digit number.