Paul Dockerty, 58, and his new wife Susan, 57, died when the Suzuki Ignis they were travelling in following their wedding in Leeds was in collision with a 38-tonne lorry on the A64 in North Yorkshire.
Their friends Joy Green, 60, and her husband Derek, 63, also died in the collision near the village of Scampston, between Malton and Scarborough, in July last year.
Coroner Michael Oakley said it was most likely that Mr Green, who was driving, was momentarily distracted by something in the car when the vehicle ploughed into the front of the lorry, driven by Vladas Monius.
The car was flipped onto its roof, leaving it with "catastrophic" damage, the coroner said.
Mr Oakley said "we will never know" what it was that caused Mr Green to veer into the path of the lorry on July 22, last year.
He said: "We have ruled out a number of possibilities and it has really come down to the fact that there has most likely been some form of distraction. What that distraction was, we will never know. Certainly, Mr Green did not brake.
"That led what was to be a very happy day to being the most horrendous day."
The coroner said the four people in the car had attended Mr and Mrs Dockerty's wedding at Leeds Register Office and were heading for week's holiday in Scarborough. Mr Green was driving and Mr Dockerty was in the front passenger seat. The two women were the rear seat passengers and all four were wearing seatbelts, the coroner said.
He said: "This was to be a day of celebration."
Lorry driver Mr Monius, who was 26 at the time of the collision, travelled from Lithuania to attend the inquest in Scarborough Town Hall.
He sat behind members of the Green and Dockerty families to hear Mr Oakley say he was blameless in the collision, which happened as he was taking frozen food from Scarborough to Nottingham.
Mr Oakley said: "He was in no position whatsoever to avoid what was a catastrophic collision with the vehicle driven by Mr Green."
Traffic Constable Terry Collins, from North Yorkshire Police, said an examination of the car's lights showed that Mr Green did not brake and was travelling at 50mph, which he described as "not an unreasonable speed" for the stretch of road.
He said there was evidence the car had travelled "quite a considerable distance" over the white line in the centre of the road.
There was no questions of alcohol or drugs being involved in the crash, the officer told the inquest.
The inquest heard that the only evidence of anything unusual before the crash was from a witness in a car behind the Suzuki who said it had drifted too close to the kerb a short distance before the impact.
The coroner recorded verdicts of accidental death on all four people in the car, who all died at the scene from multiple injuries.
He said Mr Green, who was a conservation officer, and Mrs Green, an artist, were both born in Leeds but lived in Scarborough.
Mr Dockerty, a former purchaser, and Mrs Dockerty, a former health care worker, were both born in Leeds and still lived in the city.
Their families did not want to comment after the inquest.
After the crash last year, Mr and Mrs Green's family said: "Mum and dad were amazing, enthusiastic people who did so much in their lives and were always busy doing the things they loved.
"Their lives revolved around their family and community and Scarborough where they lived and were involved in so many things in the community."