Cingular boosted its subscriber count but swung to a loss in the latest quarter as costs related to its $41 billion acquisition of AT&T Wireless piled up.While the nation's No. 1 wireless service signed up 5.7 million customers in the fourth quarter ended last month, the net gain, factoring out departing customers, was 1.8 million new users. A joint venture between SBC ( SBC) and BellSouth ( BLS), the Atlanta cell-phone shop now has 49.1 million total users. On an adjusted basis, combined AT&T Wireless and Cingular revenue in the fourth quarter inched up about 2% from a year ago to $8 billion. Cingular closed the purchase of AT&T Wireless on Oct. 26. The wireless giant improved its customer loyalty slightly. The monthly defection rate was 2.6%, compared with 2.8% in the prior quarter. Costs soared in the fourth quarter, with operating expenses running to $7.3 billion, including $245 million for merger integration. The company says integration is on plan and that so-called synergies are not expected to cut operating expenses until later this year. The average revenue per user dipped 6% to an adjusted $49.67, in part due to the conversion of AT&T Wireless customers to lower-priced Cingular calling plans.