I'm pretty sure the answer is y=-1/4x+6
Slope intercept form of line: y=4x+21
To make an equation perpindicular to that line the slope is made opposite to the original. So the slope of 4, since it's a whole number, would become a fraction, and since the slope of 4 is positive the perpindicular line will have a negitive slope. 4 goes to -1/4.
Then you just plug in the point (4,5) to find the y-intercept.
5=-1/4(4)+b
5=-1+b
6=b
so your final answer is y=-1/4x+6