U.S. sales for each of the four largest automakers in the United States fell last month, prompting some executives to forecast a gloomy spring, a period that typically posts strong sales. General Motors and Toyota said they were optimistic about the benefits of declining interest rates and tax rebate checks that most Americans will get starting in May as part of the U.S. government's economic stimulus package. Executives at Ford Motor, however, said they did not think the industry had hit bottom yet.