How do you graph the function #f(x)=-4#?
This function is a constant so that for every value of x has always the same value of y, that is -4.
A general straight line has the following form as a function:
y = f(x) = ax+b
where a is the slope of the line and b the intercept with the y axis.
Now your function has the form:
f(x) = 0x - 4
It has slope a = 0 so it never changes (a constant) and intercepts the y axis in -4 and remember: NEVER CHANGES!!!
So, you can build your graph giving values of x say, 1, 2, 3,...and y will always be -4 giving a straight line passing through -4 and parallel to the x axis.