Difference quotient math help please?

The difference quotient is:

[f(x + h) - f(x)] / h

And then if you take the limit of h as it approaches zero, you get the first derivative of the original function.

So if you have f(x) = 4, then the output of the function regardless of input is 4, so we end up with:

[4 - 4] / h
0 / h

So as h approaches zero, this expression approaches zero.

So the slope of the line at all points is zero.

Slope = [4-4]/h = 0

f(x) = 4 is a constant function, not a number
Its value is 4 no matter what the input to f, so for example f(-5)=4, f(h)=4, f(17)=4, f(-5+h)=4, etc.
So, just use the usual formula for difference quotient.