The answer to the problem is as follows:
Well isn't the range of y=cos(u) is [-1,1]
So the max value for y=cos(u) is 1
And the min value for y=cos(u) is -1 --
So the max value for f(x)=4cos(u) is:
f(u)=4cos(u)
I hope my answer has come to your help. God bless and have a nice day ahead!