Answer
$[4,\infty)$
Work Step by Step
The function decreases on an interval when, moving from left to right, the y-values continue to get smaller for x-values in that interval. For the graph in the question, the y-values decrease for all values of $x\geq4$. Since $4$ is included in the interval, a square bracket will be used on its side while on the other end, the symbol $\infty$ will be used which is always supported by a parenthesis.
Therefore, the function is increasing on the interval $[4,\infty)$.
