HONG KONG—Hong Kong police said on Wednesday four students were arrested for “advocating terrorism” after their student union passed a motion last month mourning the death of a 50-year-old who stabbed a policeman before killing himself. Police have said an officer was stabbed from behind on July 1, while on duty with other policemen preventing protest gatherings on the anniversary of the former British colony’s return to Chinese rule in 1997. The man then stabbed himself in the chest with the knife and died later in the hospital. The policeman, 28, suffered a punctured lung but survived what Secretary for Security Chris Tang described as a terrorist act by a “lone wolf.” Shortly after the attack, a few dozen members of the Hong Kong University student union passed a motion, since withdrawn, to commemorate the 50-year-old’s death and “appreciate” his “sacrifice.” The union leaders have resigned and apologised for the …