4. (10 points) Person A leaves location $A$ to find person B, and after walking 80 kilometers, arrives at location $B$. At this point, person B had left for location $C$ half an hour earlier. Person A has been away from location $A$ for 2 hours, so A continues to location $C$ at twice the original speed. After another 2 hours, both A and B arrive at location $C$ simultaneously. What is the speed of person B in kilometers per hour?
【Analysis】First, we know that A covers 80 kilometers in 2 hours, so A's current speed and the later speed can be calculated. Then, based on A's time and speed, the distance from $B$ to $C$ can be determined. Dividing the distance by B's time gives the speed.

【Solution】Solution: A covers 80 kilometers in 2 hours, A's speed is: $80 \div 2=40$ (km/h); A's speed increases to $40 \times 2=80$ (km/h);

The distance A covers in the next 2 hours is: $2 \times 80=160$ (km/h)
B's total distance is 160 kilometers, and the time is 2.5 hours, so B's speed is: $160 \div 2.5=64$ (km/h) Therefore, the answer is: 64