Apartments at 302 Mott Street in Manhattan, where the four were arrested, are part of the investigation into Hoffman's death, according to a police source.

           The source identified suspects being investigated in connection with drugs sold to Hoffman as Juliana Luchkiw, 22, charged with criminal use of drug paraphernalia, criminal possession of a controlled substance and unlawful possession of marijuana; Max Rosenblum, 22, charged with criminal use of drug paraphernalia, criminal possession of a controlled substance and unlawful possession of marijuana; Robert Vineberg, 57, charged with criminal possession of a controlled substance and criminal use of drug paraphernalia; and Thomas Cushman, 48, charged with criminal possession of a controlled substance.

          Vineberg was found to have the actor's phone number stored in his cell phone, a law enforcement official told CNN. Police discovered the largest amount of what is believed to be heroin in his apartment, the source said.

         When police were called to Hoffman's fourth-floor Manhattan apartment Sunday, they found the actor lying on the bathroom floor with a syringe in his left arm. He was wearing shorts and a T-shirt, his eyeglasses still resting on his head, according to law enforcement sources familiar with the inquiry.

Investigators discovered close to 50 envelopes of what they believed was heroin in the apartment, the law enforcement sources said. They also found used syringes, prescription drugs and empty glassine-type bags, the sources said.

Also found in Hoffman's apartment was his personal journal, resting on a living room TV stand, two law enforcement sources said.

          Preliminary tests Tuesday showed the heroin recovered from the apartment does not contain fentanyl, a law enforcement official told CNN. More testing will be done.

Fentanyl is a powerful narcotic used to treat cancer patients' pain.

          Last week, Maryland officials said heroin tainted with fentanyl had claimed at least 37 lives since September. And last month, at least 22 people in western Pennsylvania died after using heroin mixed with fentanyl.

          While results of an autopsy will definitively reveal exactly how Hoffman, 46, died, the role heroin may or may not have played is a key part of the investigation.Police are trying to piece together the actor's movements last weekend as they look for anyone who might be linked to the drugs that apparently killed him.

          On Tuesday, a law enforcement source told CNN that the night before Hoffman died, he withdrew $1,200 from a grocery store ATM near his apartment.

Hoffman got the money in six transactions Saturday night, according to the source.

         A witness told investigators he saw the Oscar-winning actor talking to two men wearing messenger bags about 8 p.m.

Police are also reviewing surveillance video, including video of a restaurant where Hoffman had brunch Saturday morning with two people.

In a 2011 interview with "60 Minutes," Hoffman discussed his past struggles with drug and alcohol addiction.

"Anything I could get my hands on, I liked it all," he said.

Fear, Hoffman said, made him sober up.

          "You get panicked. ... I was 22 and I got panicked for my life. It really was, it was just that," he said. "And I always think, 'God, I have so much empathy for these young actors that are 19 and all of a sudden are beautiful and famous and rich.' I'm like, 'Oh my God. I'd be dead.'"

But last year, Hoffman said he'd fallen off the wagon, started taking prescription pills and slipped into snorting heroin, according to TMZ.

Magazine writer John Arundel said he met the actor at the Sundance Film Festival in Utah two weeks before his death.

"I said, 'What do you do?' And at that point, he took off his hat and he said, 'I'm a heroin addict,'" Arundel said.

"Didn't look like he was (joking). Seemed like he was having one of those 'coming to God' moments -- where it just stuck him as, 'this is the revelatory moment.'"

          But actor George Clooney said he had dinner with Hoffman a few months ago, and he seemed fine. "I have to say he seemed in pretty good shape," Clooney said. "I mean, there's no way to explain it."

          Filmmaker Chris Barrett interviewed Hoffman January 17 at the Sundance Film Festival. "He didn't look well at Sundance. His skin color was very pale, but he wasn't disheveled as some media was reporting," Barrett told CNN.

Family and close friends of the actor will hold a private funeral service in New York. Plans are also under way for a memorial service later this month. No information on the dates was available.

          On Wednesday night, the famed Broadway theater district will dim its marquees for one minute at 7:45 p.m. in Hoffman's memory.

Hoffman appeared on Broadway three times.