You want to do exactly what this says, use the graph. If you were to just plug the 4 into the equation, you would end up with an undefined fraction. However, if you graph it in your calculator, take a look at what is going on with the graph as the x approaches 4 to easily find your limit. Remember to put parenthesis around the denominator when putting it into the calculator so it will graph correctly. Also, remember to write your asymptote as a linear equation, not just a number.