Mr Cheng, a long-serving accountant with police, was gunned down on October 21 last year as he left police headquarters in Parramatta.

The shooter, 15 year-old Farhad Jabar, was then killed by police guarding the headquarters.

The men — aged 18, 20, 22 and 23 — are all accused of conspiring to carry out the terror attack which led to Mr Cheng being shot dead.

The 20-year-old is also accused of organising the travel of 15-year-old gunman Farhad Jabar’s sister to Syria on the day before the attack.

The 18-year-old is charged with directing the activities of a criminal group and supplying the pistol to Jabar, while a 23-year-old from Merrylands and a 22-year-old from Marsfield will be charged with membership of a terrorist organisation.

Speaking to media on Wednesday afternoon, NSW Police Deputy Commissioner Catherine Burn said the four men were thought to be “principal” actors in the crime.

“It is important that the community is assured that as of today there is no new specific threat,” she said.

“However, we will continue to investigate any threats that we come across.”

She couldn’t rule out further charges but said the four men had been their major focus.

Police could not say where Jabar’s sister was now.

All four men have previously been charged with other crimes and are expected to appear in Goulburn Local Court via video-link tomorrow.