Investigators zeroed in on the four after a tipster, responding to publicity about Hoffman's death, told police he had seen Hoffman at the lower Manhattan apartment building where they were arrested on Tuesday and he believed that's where Hoffman got the heroin, the officials said.
After obtaining search warrants for two apartments in the building, police found about 350 packets of heroin in one of them, the officials said. They also learned from phone records that one of the suspects had Hoffman's number, strengthening the theory that they may have supplied him with drugs, the officials said.
Some of the packets found in Hoffman's apartment were variously stamped with the ace of hearts and others with the ace of spades. Those found in the building where the arrests occurred had different brand names, including Black List and Panda, the officials said.
The officials weren't authorized to speak about evidence in the ongoing investigation of the death and spoke to The Associated Press on the condition of anonymity.
Police were still waiting for a cause of death for the Oscar-winning actor from the medical examiner's office, which said on Wednesday that more tests were needed.
There was no timetable for Hoffman's autopsy to be finished, said medical examiner's office spokeswoman Julie Bolcer, who declined to discuss the pending tests. Toxicology and tissue tests are typically done in such cases.
Hoffman, 46, was found dead Sunday with a needle in his arm, and tests found heroin in samples from at least 50 packets in his apartment in Manhattan's Greenwich Village, law enforcement officials have said.
The four suspects face charges of criminal possession of a controlled substance. Two also face charges of criminal possession of drug paraphernalia.
Courts have found that under state law drug dealers can't be held liable for customers' deaths.
A 1972 state appellate division case found a dealer can't be found guilty of manslaughter or criminally negligent homicide for selling heroin and syringes to a customer who later dies because, the court ruled, legislation enhancing punishment for drug crimes didn't redefine homicide to include the sale of an illicit drug that results in death.
And holding a drug dealer criminally liable for a customer's overdose death could prove difficult for the district attorney's office, said James Cohen, a Fordham University School of Law professor who runs a clinic that represents federal criminal defendants.
"It's not just enough that you know, if you will, theoretically or academically, that heroin could kill," he said.
Former police detective Scott Prendergast, who worked on the high-profile investigation into the 1996 heroin overdose death of Jonathan Melvoin, a keyboard player touring with the rock band Smashing Pumpkins, said it's not uncommon for investigators to track down dealers following suspected overdose deaths especially when the drugs are stamped with telling names.
Investigators have determined that the "Capote" star made six ATM transactions for a total of $1,200 inside a supermarket near his home the day before his death, law enforcement officials have said. Investigators are examining a computer and two iPads found at the scene for clues and recovered syringes, a charred spoon and various prescription medications, including a blood pressure drug and a muscle relaxant, law enforcement officials have said.
A private funeral for relatives and close friends of Hoffman is set for Friday, and a larger memorial service will be held later this month, his publicist Karen Samfilippo said.
On Wednesday night, Broadway theaters were to dim their lights in memory of the Tony Award-nominated actor, and members of the theater community planned to hold a candlelight vigil.
Associated Press writer Jennifer Peltz contributed to this report.