Police have charged a fourth man in the alleged kidnap and murder of a Sydney grandfather who authorities say was taken by mistake.

The charges come two months after widower Chris Baghsarian, 85, was kidnapped in a pre-dawn raid at his North Ryde home. His body was found near a golf course about two weeks later.

A teenager, 19, was arrested on Tuesday and charged with murder and take or detain in company with intent to ransom occasioning actual bodily harm. He was refused bail and is due to appear in court on Wednesday.

Three other men, all in their 20s, are also facing similar charges with more arrests expected after investigators earlier said up to nine people may have been involved.

Baghsarian was taken from his home at around 05:00 local time on 13 February, with footage showing what appeared to be several men bundling the elderly man into an SUV before driving off.

Baghsarian's family and police said he had no criminal connections and local media reported that the alleged attack may have been aimed at a family living nearby with suspected links to organised crime.

In the days after the alleged kidnapping, images and videos purportedly showing the grandfather, seriously injured and wearing the pyjamas he was kidnapped in, were claimed to have been circulating among Sydney's underworld.

Police made several public pleas for Baghsarian's return, given he had health issues and needed daily medication.

In statements at the time of his kidnap, Baghsarian's family said he was "deeply loved, gentle, and the kindest person we know - someone who would never hurt a fly".

A day after Baghsarian's body was found in north-west Sydney, police arrested and charged two men, aged 29 and 24, with kidnap and murder.

Last month, a third person, 23, was arrested and charged with multiple offences including kidnap, murder, and firearms and drugs offences, with police describing him as a "major player" in Baghsarian's case.