4. (10 points) The distance between places $A$ and $B$ is 600 kilometers. Two people, Jia and Yi, start cycling from $A$ to $B$ at the same time. Jia rides 40 kilometers per day, while Yi rides 60 kilometers per day but rests every other day. At the end of the $\qquad$th day, the distance Yi has left to $B$ is twice the distance Jia has left to $B$.