Four of the nation's biggest banks yesterday slashed their prime lending rates from 13 to 12 percent, the lowest level for that key business interest charge since September 1980.

The reduction puts the prime 4 1/2 percentage points below the level that prevailed last June. It is a direct response to an apparent easing in the tight money policies of the Federal Reserve.

The government, meanwhile, lowered the interest rate on FHA and VA mortgages by one percentage point to 12 1/2 percent, the lowest it has been since August 1980. It was the fourth decline since early August, when the rate stood at 15 1/2 percent. Conventional mortgage rates have been falling as well and now are around 14 to 14 1/2 percent, according to lenders.

Last week there were reports that the Federal Reserve had shifted its focus, at least temporarily, from the growth of the money supply to the level of interest rates because it feels that inflation is less of a threat to the economy than the severe recession that has boosted unemployment to a postwar record.

Federal Reserve Chairman Paul A. Volcker, however, has said that the nation's central bank has not altered its basic money and credit policies.

Nevertheless, late last Friday the Fed cut its own discount rate, which is the interest that financial institutions pay to borrow from the central bank, to 9 1/2 percent from 10 percent, and bankers and investors alike appear convinced the Fed wants rates to go lower still.

White House spokesman Larry Speakes yesterday declined to comment on whether the Fed has changed course, saying "we don't normally tell the Fed what to do."

But when President Reagan was told yesterday of the prime rate reduction, he said he was "mighty pleased to see another burst of sunshine this morning." He said the full-percentage-point cut was "one more confirmation that we're on course."

Morgan Guaranty Trust Co., the nation's fifth-biggest bank, was the first institution to trim the prime yesterday, followed by Bank of America, the nation's largest; Manufacturers Hanover, No. 4; and Chemical Bank, the sixth-biggest. American Security Bank, Washington's second-biggest, said it also would cut its prime lending rate from 13 to 12 percent.

Treasury Secretary Donald Regan said that he thinks the prime rate will level off at 12 percent for a while, but called it "a pretty good rate." He said the rate was "better than 99 percent of the population expected by mid-October."

Richard Peterson, chief economist for Continental Illinois National Bank, the nation's biggest corporate lender, said the declines in the prime rate since last June make it easier for troubled businesses to make money and stay current on their debt payments. "It means a lot to them," Peterson said.

The Federal Reserve, which sets the nation's money and credit policies, permitted interest rates to climb to record levels in 1980 and 1981 in an attempt to fight the double-digit inflation that had plagued the nation since 1978. But the Fed came under heavy criticism from Congress and many economists, who charge that the Fed's tight monetary policy has triggered two recessions since 1980.

The U.S. unemployment rate last month was 10.1 percent, the highest it has been since 1940, when the nation was emerging from the Great Depression. Businesses are failing at a record rate, and most forecasters anticipate that the economy will recover slowly for the next 12 months.