One eyewitness, 23-year-old Liam Llewellyn from Caldicot, was on his way home when he approached the crash eastbound around St Julians.
He told radio station Capital FM South Wales he was shocked at the number of motorway travellers who tried to continue their journeys without stopping to help, despite the obvious scale of the crash.
Mr Llewellyn said he was initially just one of four members of the public to offer assistance.
My first instinct was to call 999, I managed to get through to emergency services, but the worst bit was everyone just appeared to stay in their vehicles, and no one was willing to help.
As I was on the phone to 999, I asked them do they want me to get out of the vehicle and assess, so then at that point I went to the bus to see the driver, who was in a bad state.
Because the car had actually hit the central reservation, the lady in the vehicle wasn't being attended, so I assessed her and she was OK, but obviously quite distressed.
I don't want to go into details of what I saw, but it was something I've never seen before in my life.
[The students on the bus] didn't speak much English, they appeared to be on tour, but it was just making sure they got to safety as well, get their belongings off, and assessing them, the hand gestures, asking 'are you OK'.
I've always been brought up in an environment where you need to look after people and they'll look after you and I like to think that if I was ever in a situation like that, people would come and help me as well.
– Liam Llewellyn, M4 crash eyewitness
I wouldn't class myself as a hero at all, you just do what you got to do to make sure someone's safe.
But a police officer said to the people who helped: 'It's a pity there aren't more like you'.