An explosion rocked a dynamite factory early today, killing at least four people and leaving one missing, local officials said. The explosion at the factory in Ablon, near this port city in the Calvados region of Normandy, was the third there since 1985. Today's blast occurred shortly after the factory opened at 9 a.m., police said. The regional prefecture said the explosion apparently occurred during the handling of nitroglycerine. An explosion at the factory in March 1985 killed three people. An explosion in March 1987 destroyed a storage area where nitroglycerine was kept but caused no injuries. The factory was closed for several months following that blast.