A four-day strike at Petrobras gathered steam on Wednesday, cutting crude and natural gas output from the No. 2 South American oil producer and threatening to become the most disruptive walkout at the state-run oil company in 20 years. On Wednesday, Petroleo Brasileiro SA, as Petrobras is formally known, said in a securities filing late Wednesday that oil output in Brazil was about 140,000 barrels a day, or 6.5 percent below pre-strike levels of about 2.1 million barrels a day. Using contingency plans management restored production that was cut by as much as 273,000 barrels a day, or 13 percent...