The car broke through the railings on a bridge over the canal in Datteln, North Rhine-Westphalia, and hit the water at 6.50pm.
Emergency service workers found three bodies in the wreckage of the vehicle and another was discovered nearby.
The victims were a 76-year-old man from Dortmund, his 70-year-old wife and a couple from Waltrop, both aged 74, a police spokesman confirmed.
The 74-year-old man was driving the Volvo and police said in a statement on Monday a preliminary investigation showed he had a medical emergency at the wheel.
According to the WAZ newspaper, this emergency was a heart attack.
The Volvo was lifted out of the water with a crane and floats in the early hours of Monday morning.
A police spokesperson said the crash appeared to have been caused by a "chain of unfortunate circumstances".
The investigation is continuing.