First, when x=0 and y=-4, we can get c= -4.

According to Vieta's theorem:

△ =4b？ + 16a=0。 . . . . . . ①

It can be judged from ① that A must be less than 0, and it is inferred that this quadratic function has the largest downward opening.

Find the maximum point of symmetry axis and change the function into the form of symmetry axis:

f(x)=ax？ +bx-4=a(x+b/2)？ -4-b？ /4a

So its maximum value is -4- b? /4a .。 . . . . ②

Bring ① into ②, and the maximum value is -3.