4. For the rectangular prism $A B C D-A_{1} B_{1} C_{1} D_{1}$, the length, width, and height are $a$, $b$, and $c$ respectively, and $a > b > c$. The shortest path from $A$ to $C_{1}$ along the surface is $\qquad$ .
$\sqrt{a^{2}+(b+c)^{2}}$