  An acyclic edge coloring of a graph $G$ is a proper edge coloring such that
no bichromatic cycles are produced. The acyclic chromatic index $a'(G)$ of $G$
is the smallest integer $k$ such that $G$ has an acyclic edge coloring using
$k$ colors. Fiam${\rm \check{c}}$ik (1978) and later Alon, Sudakov and Zaks
(2001) conjectured that $a'(G)\le \Delta + 2$ for any simple graph $G$ with
maximum degree $\Delta$. Basavaraju and Chandran (2009) showed that every graph
$G$ with $\Delta=4$, which is not 4-regular, satisfies the conjecture. In this
paper, we settle the 4-regular case, i.e., we show that every 4-regular graph
$G$ has $a'(G)\le 6$.
