Four people were killed and 18 others injured on Sunday in a suicide bombing in Iraq's capital Baghdad, police said.A suicide bomber detonated his explosives-laden vest at a tent set up near a school building by a group of Shia Muslims as part of the preparations to observe one of their major rituals to honour the holy month of Muharram beginning next week, Xinhua news agency cited the police as saying.No group has so far claimed responsibility for the attack.