If you plug four in, you get 0/0, so you need to apply L'Hopital's Rule and take the derivative of the numerator and the derivative of the denominator (separately, not using the quotient rule). This gives you the limit as x approaches 4 of 3 x^2 or 48.
Edit: Hope that helped! Also, what did you use to get the expression as a nice little picture with all the symbols? I'm new to this.