Sum probability

The numbers $a$ and $b$ are given such that $0 \leq a \leq 4$ and $0 \leq b \leq 8$. The probability that $a + b \leq 4$ can be represented as a fraction of $\frac {m} {n}$, where $m$ and $n$ are coprime natural numbers. What is the sum of $m + n$?