Memphis residents spent their fourth day boiling water for drinking, brushing their teeth and preparing food on Monday as repair crews worked to fix broken pipes in hopes of easing the stress caused by a week of sub-freezing temperatures, snow and ice in this southern city. The city’s water company issued a boil water notice on Friday to the more than 600,000 people it serves because low pressure in the system and breaks in water mains could allow harmful bacteria to contaminate the water supply