CBS Inc. said Monday it expects to post a loss in the fourth quarter because of a weak advertising market and unexpectedly large losses on its television broadcasts of major league baseball. The company also said it expects lower earnings from continuing operations for all of 1991 compared with this year. The glum forecast comes as CBS reportedly planned a meeting with its affiliates board to propose a reduction in the estimated $160 million the network pays about 200 affiliated stations for carrying its shows. Media analysts said they expected CBS would post a loss for the last three months of 1990 and an earnings decline in 1991. Analyst Jeff Russell of Smith Barney, Harris Upham & Co. said CBS would lose about $35 million in the quarter and earnings will fall to about $205 million in 1991 from about $230 million in 1990. In reaction to the news, CBS shares dropped $4 to close at $159.75 in trading of New York Stock Exchange issues. CBS isn't the only media company suffering because of a slowdown this quarter in national ad spending growth. Capital Cities-ABC Inc., which operates the ABC Television Network, said last month as it reported a decline in profit in the third quarter that it expected a decline in earnings for the fourth quarter as well. Like CBS, Capital Cities-ABC has been hurt by losses on major league baseball coverage at its 80 percent-owned ESPN cable TV network. The third major network, NBC, is owned by General Electric Co. which does not report separately on its broadcasting subsidiary's performance. CBS's unspecified loss projected for the fourth quarter would be its first excluding special items since the company lost $20 million on continuing operations in the fourth quarter of 1986, spokesman Kewith Fawcett said. CBS earned $59.5 million, or $2.31 a share, in the fourth quarter of 1989. This was the first year of CBS's four-year baseball contract which allows it to carry several regular season games along with the league championship series, World Series and All-Star game. While the network reportedly expected to lose about $35 million in the first year of the $1.06 billion contract, the losses are said to have run much higher. CBS Sports President Neal Pilson has said only that the loss on baseball was under $100 million. On Monday, CBS said losses on baseball were ``higher than anticipated'' as the result of soft demand in the ad marketplace for time on sports programs and because the World Series and one of the two league championships were settled in four games instead of the maximum seven. The short series meant CBS had fewer nights of baseball on which to sell advertising. Games later in a series also tend to draw bigger audiences, making them more valuable to advertisers. CBS said its TV network would have had a loss for the fourth quarter even without baseball, because of the soft ad marketplace. It said the marketplace weakness has dampened demand for network, local TV and radio advertising, and said network ad prices have been particulartly soft for National Football League and primetime broadcasts, where CBS ratings have improved. In projecting a decline in earnings from continuing operations in 1991 compared with 1990, CBS cited the worsening economic climate for advertising and rising costs for entertainment programs and sports rights. Another reason is that the 1990 figures include profits on the coverage of the NFL Super Bowl and the National Basketball Association playoffs and championship that the network will not have rights to in 1991. ``As a result, the CBS Television Network is currently expected to show an operating loss in 1991,'' the statement said. Russell said his firm expects the CBS TV Network will post a profit of about $10 million for 1990 and lose about $35 million next year.