Alldirections. I would have pulled into the slow lane, drove slower and then pulled off the motorway.
I would have then gotten out of the car with DD and spoken to her.
I would have told her we would not be going home until she learned to behave in the car. That her behaviour was dangerous and that unless i could trust her to behave she could not get in the car and go home.
I don't shout but have a stern voice (my brother calls it the drone of death)
If a tantrum began i would ignore.
At 4 years old i think she is perfectly old enough to realise that her behaviour is unacceptable and to get what you are saying about the dangers of not wearing a belt.
In one extreme occasion i was driving at bout 6 mph, DD had gotten out of her seat yet again and so i pushed hard on the brake.
SHe fell off of the chair.
It was not fast enough to have hhurt her or caused an accident (side road with no other cars) but it did shock her enough to get back into her seat.
I pulled over and explained that hat is what happens if i have to stop the car suddenly, or if another car hit us. I said that i was only driving slowly and that if we had been going any faster she could have been seriously hurt.
Again, that was one occasion that i do not consider my proudest parenting moment.
I have also shown my children footage of crash test dummies so that they can see just what would happen to them if they don't wear their seat belts.
DD is only 6 so not that much older than your DD.
Education rather than fear.