The nation's money supply dropped sharply by $3.9 billion in the week ended April 30 for its fourth consecutive weekly decline, the Federal Reserve reported yesterday.

The money supply measure known as M1-B fell from $386.8 billion to $382.9 billion. M1-B, which includes currency in circulation and checking deposits at banks and thrift institutions, has declined by $8.8 billion since the beginning of April.

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The sharp decline began in the wake of Federal Reserve actions on March 14 to restrict credit in an effort to halt financial speculation and curb inflation.

Interest rates shot up, with the prime lending rate reaching 20 percent. In the last few weeks, short-term interest rates have plummeted, some by six full percentage points.

President Carter told a campaign audience in Philadelphia yesterday that the impact of the Fed's recent credit-tightening moves had been "many times greater than what we'd thought they'd be." But he predicted a "quick rejuvenation" in consumer spending.

As bank loan activity slackened as interest rates soared, so did money supply growth. The Federal Reserve, in response, allowed the federal funds rate, one indicator of the relative availability at banks of funds to lend, to drop sharply.

In the week ended May 7, the federal funds rate -- the interest banks charge one another for loans of reserves, usually overnight -- averaged 12.96 percent, down from 15.12 percent the week before and almost 20 percent at the beginning of last month.

One of the Fed's mid-March actions was to require money market mutual funds to set aside 15 percent of any increase in their size after March 14 in a non-interest-bearing account. According to other figures released yesterday, that step has almost halted the funds' expansion.

After explosive growth during 1979 and the first two months of this year -- assets shot up from $10.3 billion at the end of 1978 to $60.5 billion this March -- the money funds grew by only $200 million last month.

On a monthly basis, M1-B fell from $392.9 billion in February to $392.3 billion in March and $387.5 billion in April.

A broader money measure, M-2 -- which also includes money market mutual funds, savings account deposits at banks and thrift institutions, and certain other sources of bank funds -- dropped from $550 trillion in March to $1.547 trillion last month.

During most of 1979 and the first month of 1980, M-2 had been growing by between $8 billion and $10 billion a month.

Federal Reserve officials have indicated they plan to continue to supply reserves to the banking system, letting interest rates decline until loan activity picks up again and money supply growth resumes.

Between last October -- when the Fed changed its procedures for controlling money supply growth -- and mid-March, it exactly hit its target for growth at a 4.5 percent annual rate. Now the Fed has dropped well below that goal, with M1-B falling at a 1.5 percent annual rate from January to April, the Fed reported.