Accused Colombian drug smuggler Marcos Cadavid declined to take the witness stand yesterday, abruptly ending testimony in his four-day trial at U.S. District Court here on a charge of conspiracy to import and sell cocaine.

"I do not wish to testify," Cadavid, speaking through an interpreter, told U.S. District Judge Thomas F. Hogan after federal prosecutors completed their case.

The jury is expected to begin deliberating today after lawyers present their closing arguments.

The jurors have been sequestered since the trial began on Monday, and the federal courthouse has been placed under heavy security because of unspecified death threats from South American drug dealers.

According to U.S. law enforcement officials, the dealers were angered by Cadavid's extradition from Colombia two months ago, under a new treaty with the United States, to stand trial here.

Police wearing bulletproof vests have staked out the courthouse, and officers with automatic weapons have patrolled the roof on occasion.

Federal prosecutors have made it no secret that they regard conviction of Cadavid as an important signal to Colombia that the extradition of drug smugglers to face justice here is workable.

The prosecution presented five witnesses this week who testified that Cadavid and a second Colombian, Armando (The Hammer) Marulanda, made up a Miami connection that supplied large quantities of cocaine to a Washington-based drug ring from November 1976 to January 1983.

James Kastrenakes, who has pleaded guilty in the case, testified yesterday that he visited Cadavid perhaps 70 times at a Miami apartment to help carry out the ring's drug transactions.

During one nine-month period in 1980, Kastrenakes said, he turned over between $8 million and $10 million in cash to Cadavid to finance purchases of about 175 kilograms of cocaine.

Prosecutors charged that the Washington ring paid a total of about $20 million to Cadavid and Marulanda for drugs that were distributed in the District, Virginia and elsewhere.

Kastrenakes, who testified that he oversaw the collection of cash from the ring's couriers, said he often was forced to straighten the group's money because "the denominations would be all messed up." He said he separated the bills, put on "new rubber bands" and saw that the cash was stacked neatly. "That way," he testified, "I knew it was my money."

Kastrenakes said he usually sat in his van until he saw Cadavid carrying "the product" in boxes into the Miami apartment and then went inside to complete the purchases with hundreds of thousands of dollars in cash.

If convicted, Cadavid faces a maximum penalty of 15 years in prison and a $25,000 fine.