International Paper Co. said it earned $44 million in the fourth quarter of 1985 compared with a $74-million loss a year earlier, attributing the gain to a spin-off of 6 million acres of timberland. IP, one of the nation's largest landowners, formed a proprietary partnership in 1985, selling shares of some of its timberland to the public. In the fourth quarter a public offering generated a pretax gain of $56.5 million. This was partly offset by a pretax charge of $12 million related to closings of field offices and a pulp unit at its Natchez, Miss., mill. In the fourth quarter of 1984, the company took a $155-million pretax charge primarily for anticipated losses from the sale or closing of certain wood products plants, and expected losses on some West Coast timberland leases.