What is religiously binding upon a driver who caused an accident leading to the death of four people due to falling asleep while driving, and is it true that the forgiveness of the victims' families regarding the blood money (diyya) negates the obligation of fasting for him?

If the accident occurred due to the driver's negligence or transgression, he is liable for what was damaged in terms of lives and property. He also owes an expiation for each person who dies in the accident, which is a right of Allah Almighty and is not waived by forgiveness. As for the blood money (diyya), it is a right of the heirs of the deceased and is borne by the killer's 'aqila (male agnates), and it is waived by forgiveness. The expiation multiplies with the number of deceased. If there was no negligence or transgression on the part of the driver, then nothing is due from him.