American Motors Corp., the fourth largest U.S. automaker, reported reduced losses for 1981 yesterday but said it had a larger shortfall in the fourth quarter than in the same period a year before.

The company, which is almost 50 percent owned by Renault of France, had a loss of $136.6 million in 1981($2.44 a share) compared with $200.8 million ($6.10) in 1980. In the fourth quarter, it lost $47.2 million (85 cents) compared with $29.0 million (81 cents) in the year-ago period.

"The past year was one of the worst in industry history," newly appointed Chairman W. Paul Tippett Jr. said in a statement.

The country's largest automobile manufacturer, General Motors Corp., has reported a $333 million profit in 1981, but the second largest, Ford Motor Co., reported a loss of $1.06 billion.

Chrysler, the third largest, has yet to report final 1981 figures.

Phibro Corp., a leading marketer and trader of commodities that recently acquired the investment banking firm Salomon Brothers Inc., said yesterday that its fourth-quarter profit fell 26 percent from a year earlier on a 16 percent drop in revenue.

For the full year, profits fell 38 percent, Phibro said.

Phibro markets such commodities as minerals, metals, fuels, sugar and grains through its Philipp Brothers division. It is also involved in the transportation and testing of raw materials. Most of the materials it markets are acquired in the open market.

For the full year, earnings slumped to $289.1 million ($4.23), from $466.8 million ($6.87) in 1980. Annual revenue rose 6 percent to $25.1 billion from $23.7 billion.