This is the equation of a parabola. Since this parabola opens downward, the vertex will be the highest point. The t-value of the vertex is found by -b/(2a).
a = -1 b = 8 c = -4
Use these to find the t-value.
Then place that value into the original expression and that answer will be the maximum possible value.
Try it; if you get stuck, post back.