By Phil Mercer
BBC News, Sydney
Policing tight-knit traditional communities can be difficult
It is believed the victims were tortured by fellow villagers in a remote highland region 400km (250 miles) north of capital Port Moresby.
Police believe they were forced to confess to witchcraft after they were stabbed with hot metal rods.
Human rights campaigners say it is not uncommon in Papua New Guinea for women suspected of witchcraft to be killed.
These four women had been accused by fellow villagers of using sorcery to cause a car crash in which three prison guards died.
A senior police officer said it appeared the killings took place last October and that a tip-off from tribal elders had eventually alerted the authorities.
The women's bodies were found hidden in an old pit.
It is not clear if any charges will be laid.
Tight-knit communities
Superstition has always been part of life in Papua New Guinea.
Death and mysterious illness are sometimes blamed on evil curses and suspected sorcerers are often blamed and then killed.
Researchers have found that the victims are usually elderly women with little influence in the village.
Prosecuting those who kill these so-called magic makers within tight-knit communities is problematic, with potential witnesses often refusing to speak to the police.
Christianity is a powerful force in Papua New Guinea, but many people still believe in sorcery.
Those suffering from HIV and Aids are often seen as the victims of witchcraft.
Papua New Guinea has the highest HIV rate in the South Pacific - aid agencies warn of an epidemic spiralling out of control - but many people do not understand how it is spread.
In the past, some Aids victims have been thrown off bridges or dumped into graves to die.
Papua New Guinea

Villagers near the town of Goroka in the jungle-covered highlands about 400 kilometres north of the capital Port Moresby in Papua New Guinea killed four women they believed to be witches. They tortured the women by stabbing them with hot rods so they would confess to causing a road crash using sorcery. The accident had resulted in the death of three prison guards. The villagers hid the bodies in an old pit. 

With the help of some village elders, police have now discovered these murders which occurred in October of last year. BBC News reports human rights campaigners saying that it is not uncommon in Papua New Guinea for women suspected of engaging in black magic to be killed, often by hanging or burning.