We have the form
y= asin (x + b)
The function is
y = -4sin( x+ pi/2)
The amplitude = l a l = l -4 l = 4
Also.....The midline is y = 0
The amplitude is the height of the curve above the midline = 4
See the graph here :
https://www.desmos.com/calculator/o06yqc9wkk