BERLIN—Four people were injured when an old aircraft bomb exploded at a bridge near Munich’s busy main train station on Wednesday, police said on Twitter, raising the number of wounded from three earlier. The Munich fire brigade said one of the people was seriously injured. More than 2,000 tons of live bombs and munitions are discovered each year in Germany, more than 70 years after the end of World War Two. British and American warplanes pummeled the country with 1.5 million tons of bombs that killed 600,000 people. Officials estimate that 15 percent of the bombs failed to explode, some of which were buried six meters (20 feet) deep in the ground. The explosion happened as the site was being drilled to build a tunnel, police said, adding the area had been cordoned off. “There is no danger outside this area,” police said. Explosives experts were summoned to the site …