I receive updates from my Local Authority and they state that if a car share is absolutely essential, the passenger (note, the expectation is that it is only one person, and not 4) must sit in the back whilst wearing a mask and with all windows open throughout the journey.
After the person gets out, the whole car must be thoroughly cleaned.
If lippyqueen's H gave 4 people a lift, then quite honestly that was asking for trouble from the very start, whether masks were worn or not.
I do hope no one has been infected, but it was an extremely high risk thing to invite people from different families into the home where no social distancing would be possible, let alone any further safety precautions.