Answer
$${\text{ }}y = 4.{\text{ It is a horizontal asymptote}}{\text{.}}$$
Work Step by Step
$$\eqalign{
& \mathop {\lim }\limits_{x \to \infty } f\left( x \right) = 4 \cr
& {\text{The statement means that while }}x{\text{ increases to infinity the }} \cr
& {\text{function }}f\left( x \right){\text{ approaches 4: }}y = 4{\text{ is a horizontal}} \cr
& {\text{asymptote}}{\text{.}} \cr} $$
