Chemical Bank today boosted its prime lending tate to 11 3/4 percent, the highest level in four years for the bellwether of business borrowing costs. The move is expected to be followed nationwide by other banks.

Today's increase came less than a less a month after major banks started charging 111/2 percent for loans to their best corporate customers at the end of November.

The action also comes amid indications the Rederal Reserve Board has taken now steps to tihgten credit.

Chemical's decision, effective immediately, represents the 15th and perhaps last increase in the prime rate for 1978 which opened the year exactly four percentage points lower at 73/4 percent.

Although Chemical officers declined today to forecast a course for the Prime, Wall Street analysts have forecast that the upward spiral of interest rates will force banks to raise the prime regularly for the next several months.

Henery Kaufam, chief economist for the investment banking firm of Salomon Brothers, said last week that the prime reate "will reach and exceed 13 percent," and Chemical Bank's own economic report this week forecasts that interest rates won't peak until next May.

Bank officers and analysts said today that the Federal Reserve Board appears to be tightening credit in the wake of a meeting Tuesday of the Open Market Committee, which sets monetary policy.

The key federal funds rate, interest charged on overcharged on overnight lending by one bank with excess funds to another, was quoted at 10percent and higher today, after closing at 97/8 yesterday. There was speculation that the Federal Reserve has set a new target of 101/4 percent for the interbank rate, at least for the near term.

In additional to Federal Reserve pressure, commercial borrowing activity continues at a high level which also tends to push rates higher. Borrowings from large banks grew at what Chemical Bank economists called a "steamy" 25 percent rate in November, "fueled by strong demands in real estate and consumer lending and from foreign commercial banks."

During the first week of December, business borrowing s at large New York city banks jumped $350 million.

Economists at Citibank, the nation's second largest, said on e overlooked factor to explain "why rates won't peak soon" is that while interest rates now are at about the record levels of 1974- when the prime hit 12 percent for several months-the rate of inflation is somewhat lower. Thus, the real cost of borrowing money is less than in 1974.

The cost of borrowing in 1974 was 3.5 percent to 4 percent compared with 2 to 3 percent today, a "rough" measure of the difference between the inflation rate about 9 percent today and the prime, noted Citibank economist Harvey H. Segal.

Segal also noted that one unusual development related to all the recent prime rate boosts has been "the absence of all the screaming there was in 1974" about interest rates when funds wre withdrawn from savings and loan associations and money for mortgage lending became scarce.

In 1978, federal regulators have permitted savings and loan associations to offer certificates with interest rates that match closely the rates paid on treasury bills. That action has prevented a massive outflow of S&L deposits. Although mortgage money is expensive-averaging 10 percent nationwide and close to 11 percent in metropolitan Washington-it is available in states without 10 percent usury ceilings.

Although mortgage rates and consumer lending interest charges not tied directly to the prime rate, they all have been moving sharply higher. Many banks, including several in the Washington area, have begun a program of offering two prime rates-one to large businesses and another to small firms, often 11/4 percent below the prime for firms with assets of less than $1.5 million.

Chemical Bank, the 6th largest in the country, with assets of more than $30 billion said today's prime increase reflects a "continuation of the same pressures that have pushed interest rates upward throughout the fall . . . the national economy remains quire strong and loan demand is rising significantly."

At the same time, a Chemical spokesman added, inflation continues to rise at an "unacceptably high rate" and renewed pressure on the dollar appears to have resumed-pressure that the federal reserve is seeking to counter by forcing interest rates here higher.

Separately, Citibank reported that higher prices and sharp decreasing buying power have combined to give American consumers their worst case of economic jitters since the end of 1976. Two of three adults surveyed said economic conditions will worsen over the next six months.