WASHINGTON — American consumers may be paying 2 to 4 percent more for food next year, the government said Tuesday, slightly below the expected inflation rate.Agriculture Department economist James Donald said large meat supplies should dampen food price inflation, which has moderated from the 5.8 percent rate in 1990. Mr. Donald said the 1991 food inflation rate was estimated at just over 3 percent, below the expected rise in the Consumer Price Index of more than 4 percent. Despite slight rises in food prices and a predicted increase in US agricultural exports suggest better times for farmers, Donald said modest increases in prices for fuel, fertilizer, pesticides and labor should cut into their receipts. That should leave farmers next year with a net cash income of $52 billion to $57 billion, down from the estimated $58 billion for 1991 and well off the record $62 billion set in 1990. Agricultural exports for fiscal 1992, which started in October, should reach $39 billion, up 4 percent from 1991. Higher grain prices and increased volume of wheat and oil seed product exports should boost the value, Donald said. Imports of farm products, meanwhile, should slip, especially of livestock and animal products and some tropical products including sugar.