x = -4 is not undefined because f(x) = x+4 is in the top part, the numerator, not the denominator. So the value that makes f(x) = x + 4 zero does not result in a division by zero.
Indeed, if we try to evaluate (f/g)(-4), then we get
which does not involve a division by zero.
However, attempting to evaluate (f/g)(3) leads us to
which involves a division by zero, and divisions by zero are undefined