Chapter 13 - A Preview of Calculus: The Limit, Derivative, and Integral of a Function - Section 13.1 Finding Limits Using Tables and Graphs - 13.1 Assess Your Understanding - Page 896: 23

$13$.

As we can see by the attached graph (please refer to the green line), as $x$ gets closer and closer to $4$ from the left and the right, the value of the function (the y-value) gets closer and closer to $13$. Thus the limit is $13$.