A four-day shopping spree starts today, a cacophony of cash registers that will determine not only what's under the tree on Christmas morning, but also what's under the bottom line when the nation's merchants add up the year.

If business between now and Monday is as good as expected, retailers will breathe a sigh of relief that the consumer has come through again, despite soaring petroleum prices, the threat of a recession and world-wide malaise.

And if business is not so good, most people in the retail trades will breathe a sigh of relief anyway, just for surviving the longest, loudest, most loathsome work week of the year.

The next four days "are going to be insane," predicts Woodward & Lothrop Chairman Edward K. Hoffman. "It had better be insane, or we will be."

So far, agree Hoffman and most merchants, business has been good, but by no means great. Sales are ahead of the same date a year ago, but inflation and the quirks of the calendar account for much of that.

To a greater extent than usual, the stores are looking for the final days to make their year. That's been the pattern for the past two or three Christmases. Sales start slowly but end with a frenzy that threatens to overload the credit card verification lines and turn the wrapping room into a rubber room.

This year, especially, that spree will be spurred by what the head of Washington's biggest department store chain calls "a highly promotional atmosphere."

That means longer store hours, heavy advertising, plenty of price cuts and piling up promotional merchandise so the customers can't avoid it.

Consumers are responding to that approach and their shopping habits indicate they are buying a mixture of baubles and basic goods.

Children's clothes and "snuggle sacks" -- for keeping warm when the thermostat's at 65 degrees -- are going like gangbusters at Montgomery Ward, a spokesman said, but so are microwave ovens and gold jewelry, not exactly necessities.

The outerwear business, which was not so hot when the weather was warm, has turned around dramatically now that winter has finally arrived in Washington. The new quilted coats for women are leading the sales, selling well from discount stores to boutiques.

"Business is very, very good. All of our divisions are sharing in the strength," said Ted Trimmer, corporate secretary of Garfinckel, Brooks Brothers, Miller & Rhoades Inc., the Washington-based retail conglomerate.

"Sales are surprisingly strong," he added, "I don't think we anticipated the strength, in view of the general economic climate."

Trimmer said the Garfinckel companies "are now looking for a respectable year," after falling short of profit targets earlier in 1979.

Hoffman said the projit picture has also improved at Woodies and stocks of merchandise are not so heavy that extraordinary clearance sales will be needed in January.

Hoffman noted that Woodies' November business was substantialy ahead of last year, but part of that gain was "stolen" from December. That's because the way retailers count their sales; there were four shopping weeks in December last year, but only three this year. Thus December sales may not look good by comparison with 1978, the best Christmas season ever.

While December is all that retailers are worrying about now, January is looming ominously on their horizon.

When the December fuel bill arrives, the latest OPEC oil price increase filters through to the gas pumps, and social security deductions start coming out of paychecks again, the retail recession could begin.