Answer
The velocity decreases at a rate of $8$ x $10^{-5}\ cm/sec$.
Work Step by Step
$v = 6.4 * 10^{-8} - 0.001r^2$
To find the rate of change, we have to find the derivative f the function which we can do using the power rule:
$v' = -0.002r$
Now we substitute r=0.004 into the equation to find the rate of change at that specific point:
$v' = -0.002(0.004) = -0.00008$
To interpret this answer, we note that the number is negative and so the velocity decreases at a rate of $8$ x $10^{-5}\ cm/sec$
