The smash happened in County Down, just a few miles away from the border with the Irish Republic.
It is the biggest single loss of life for police in Northern Ireland since the end of the Troubles.
Britain's roads are amongst the safest in the world though, within Britain, deaths on Northern Ireland's roads have increased in recent years.
Four police officers killed in freak road accident in Northern Ireland
By Stephen Wright
23rd November 2008
Four police officers were killed in a horrific car crash while answering a routine emergency call to assist a violent arrest.
It happened as the officers raced to assist colleagues in another police unit trying to detain a man over a minor offence, believed to be drink-related.
The four wheel drive Shogun vehicle was reduced to a burnt-out wreckage with only the driver's door recognisable as a police car.
The remains of the burnt out police car are at the centre of the investigation
The car exploded in flames after it skidded into a stone wall, rupturing the fuel tank, on a notorious road.
Live ammunition which was being carried by the officers exploded on impact, meaning they had virtually no chance of escape.
One of the dead officers had just become a father and it is the single worst loss of life for the Police Service of Northern Ireland since the end of the Troubles.
The horror smash happened between Warrenpoint and Kilkeel in Co. Down, as the officers - two full time policeman and two part-time colleagues - were answering an urgent call for assistance from colleagues trying to arrest a man nearby.
The exact cause of the accident is still being investigated but early inquiries suggested that poor weather conditions and excess speed may have resulted in the car spinning out of control.
The PSNI said it appeared no other vehicle was involved in the accident and that there was no suggestion that there were any terrorist or paramilitary links.
All four officers were from the Mourne area of Co Down and two of them had young families.
The crash site has been sealed off as accident investigators conduct a fingertip examination of the scene and prepare to remove the burnt-out shell of the vehicle which will undergo a detailed forensic investigation. It is hoped the vehicle's 'black box' recorder can explain the cause of the crash.
One eyewitness said: 'Just from looking at the mangled wreck of the vehicle it is obvious that these officers did not stand a chance of getting out alive. Nobody could have survived such carnage. It is too horrific for words and the lives of four families have been turned upside down in the run up to Christmas. One just can't begin to imagine what these families are going through right now.'
Chief Constable Sir Hugh Orde, who was amongst the first to visit the scene of the tragedy, said he was 'shocked' at the carnage.
Terry Spence, Chairman of the Northern Ireland Police Federation said: "Two of these officers had children and coming up to Christmas this will be extremely difficult for their extended families but for all of these officers there will be mothers, fathers, brothers, sisters, the police service in general and in particular the colleagues who served with these men so it will be a very very difficult time foe everyone involved.'
Democratic Unionist Party assembly member for the area Jim Wells visited the scene and said the deaths were particularly poignant in the run up to Christmas.
Mr Wells said: "o make matters even more tragic, a very young baby has been born to the wife of one of those officers.
'I know that the circumstances of the other officers are equally tragic so this is an awful day for the whole community.'