CHINA (SHANGHAI INTERNATIONAL AIRPORT)  - A small explosion at a check-in area of Shanghai's main international airport injured four people Sunday but did not cause any disruption to flights. The blast at Pudong International Airport, China's second-busiest, occurred at around 2:20 p.m. and appeared to have been caused by some sort of homemade explosive.  Four injured people were taken to a hospital, conditions unknown. There was no disruption to flights into or out of the airport, the management said, although police and internal security troops cordoned off the area and checked for additional explosives.