Southwest Airlines reported a $78 million profit for the fourth quarter, almost a 50 percent drop from its $152 million profit posted in the same period in 2011.The Dallas-based carrier said revenues grew slightly in the quarter to $4.2 billion even as Southwest reported more empty seats on its planes for the quarter. In the fourth quarter, the average fare grew 5.4 percent to $148.02.For the full year, Southwest reported a profit of $421 million for its fortieth consecutive profitable year. Revenues increased 9 percent to $17.1 billion offsetting higher operating costs, which also rose by 10 percent."These solid earnings were achieved despite significant efforts and costs related to critical strategic initiatives," said Southwest chief executive Gary Kelly. "I expect these initiatives to produce substantial returns over the next several years."The carrier retrofitted 259 Boeing 737-700 aircraft in 2012 with its new cabin interior that also added six more seats to each plane. Southwest expects to have its entire fleet of 372 Boeing 737-700 aircraft redone by June and 78 of its older Boeing 737-300 aircraft retrofitted by the end of the year.Excluding one-time accounting items, Southwest said its fourth quarter net income was $65 million, or 9 cents a share, which beat Wall Street analysts estimate of 8 cents per share, according to First Call.Southwest executives will discuss its earnings with investors on a conference call at 10:30 a.m. on Thursday.