AMR Corp.'s American Airlines and Delta Air Lines Inc. raised their leisure fares 4%, while UAL Corp.'s United Airlines matched that hike and also increased business fares, but the carriers could again drop the price hikes if Northwest Airlines Corp. doesn't match them, said Tom Parsons, editor of Bestfares.com. The major carriers have raised leisure fares nine times this year, only to roll back the increases when Northwest didn't fall in line. A Northwest spokesman declined to comment other than to say that the airline had taken no action on fares Friday. The St. Paul, Minn.-based airline already has some fare sales in place that started after the end of a two-week strike that shut down the airline. While some analysts expected Northwest to initiate a huge fare sale after the strike to lure customers back, the carrier's executives said recently that demand was returning without the added incentive.