The largest U.S. refinery strike in 35 years entered its fourth week on Sunday as workers at 12 refineries accounting for one-fifth of national production capacity were walking picket lines. Sources familiar with the negotiations said talks may resume by mid-week to end the walkout by 6,550 members of the United Steelworkers union (USW) at 15 plants, including the 12 refineries. The refinery work stoppage began on Feb. 1 when talks for a new three-year contract between the USW and lead oil company negotiator Shell Oil Co broke down.