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.