Sajjad Ahmad, 37, was the driver of a Mercedes which was involved in a collision with a van between Polmont and Linlithgow on Friday evening.
His death on Tuesday followed his wife, Saadia, 37, who died at the scene and their nine-year-old daughter, Ayesha, who died in hospital on Saturday.
Mark Hansen, 42, the driver of a white Vauxhall van, died at the scene.
The Ahmads' two sons remain in hospital.
The motorway was closed between Junctions 3 and 4 southbound until 04:00 on Saturday for a police investigation.
Sgt Scott Sneddon, of Police Scotland, said: "Our sincere condolences go to the Ahmad family, who have asked that their privacy be respected at this tragic time.
"Our inquiries continue to establish the full circumstances surrounding this collision, and I'd like to thank the witnesses who have so far come forward.
"Anyone who witnessed the collision, saw either of the vehicles shortly beforehand, or has any information about this and has not yet been in contact is urged to get in touch."
A fourth person has died following a serious crash on the M9 in West Lothian last week.