The nation's Gross National Product declined at a 5.2 percent annual rate in the fourth quarter of 1981 as both business and individuals cut back on spending and unemployment soared, the Commerce Department reported yesterday.

Economic experts both inside and out of the Reagan administration predicted a further decline in the GNP and even higher unemployment in the first quarter of 1982.

Treasury Secretary Donald T. Regan yesterday forecast a further decline in GNP in the current quarter at an annual rate of about 2 percent. The GNP is the measure of the nation's output of goods and services.

Many private economists are more pessimistic. Allen Sinai of Data Resources Inc. told a congressional panel yesterday that "it is clear the bottom of the recession has not been reached . . . and the situation appears quite bleak." He warned that the economy "runs the risk of a major collapse, unprecedented in the postwar period" if there are not major policy adjustments.

Deputy Commerce Secretary Joseph Wright said the preliminary GNP figures released yesterday "confirm that the recession, which began in July, had an impact on most major sectors of the economy."

In 1981 as a whole, real GNP rose by 1.9 percent after falling by 0.2 percent in 1980. Most of last year's increase came in the first quarter when output grew by 8.6 percent after accounting for inflation. In the second quarter, GNP fell by 1.6 percent, followed by a rise of 1.4 percent in the third quarter.

Regan also said the consumer price index would probably show a rise of just over 9 percent for 1981 when it is released Friday.

With the continuing signs of recession, administration officials yesterday continued to grumble about the money policies of the Federal Reserve Board. Regan said that last year's money growth was less than the administration had allowed for in its plans, and that "this did not help us in our economic recovery program."

Earlier this week, however, both the treasury secretary and the president were criticizing the Fed for allowing too rapid an increase in the money supply in the last few weeks. The president told a news conference Tuesday that the recent upsurge in the money supply sent the "wrong signal" to money markets.

Regan said yesterday the Fed's money supply tools "need to be sharpened, perhaps redeployed and certainly made more precise." Erratic money growth last year, "following an unsettling money supply rollercoaster in 1980, kept financial markets nervous," he commented at a Washington luncheon.

Deputy Treasury Secretary R. T. McNamar yesterday added his voice to the chorus of administration complaints about the Fed, saying the jump in the money supply since November will threaten to rekindle inflation if it continues. "Interest rates have temporarily risen in response to that danger," he said.

Record high interest rates last year brought on the present recession, and some economists fear a renewed rise in rates could choke off any recovery. However Regan predicted the economy would grow by 1 percent overall this year, after allowing for inflation, with a strong growth rate in the second half of the year.

He said this assumed the Fed hits its target for money growth this year so there is "sufficient money to allow" recovery.

A breakdown of the fourth quarter GNP report showed personal spending, after taking account of inflation, fell by 1.3 percent at a seasonally adjusted annual rate while business fixed investment dropped by 2.9 percent and exports fell by 2.6 percent on the same basis.

Most of the drop in personal and business spending was due to reduced purchases of autos, the Commerce report said.

Companies continued to build up inventories in the quarter, but at a slower rate than during the third quarter, thus pulling down the GNP.

The federal government was the only sector of the economy that increased spending in real terms. Unexpectedly large purchases of farm products by the Commodity Credit Corp. pushed up non-defense spending, and defense spending also rose in real terms, the Commerce Department said.

Although President Reagan has denied that the recession is being used to fight inflation, it is having the effect of lowering wage and price rises. In the fourth quarter of last year the broadest measure of inflation, the fixed weight GNP price index, increased by 8.3 percent at an annual rate.

"Excluding the federal pay raise, the rise was only about 7.1 percent," Wright said, adding "this represents a marked reduction from the average inflation rate of 9.3 percent during the first three quarters" of 1981.

In a related matter, Regan said yesterday that the president would again propose dropping the tax exemption for industrial revenue bonds, and would suggest some other "changes in the tax code" but would fight any change in the personal and business tax cuts enacted last summer.