In the past twenty years hundreds of infants and young children have died after being left in vehicles, usually by accident. When turning the vehicle off, drivers of the M5 are reminded to check the back seat if they opened the rear door before starting out. The S4 doesn’t offer a back seat reminder.
Over 200 people are killed each year when backed over by motor vehicles. The M5 has standard Active Park Distance Control that uses rear sensors to monitor for objects to the rear and automatically applies the brakes to prevent a collision. The S4 doesn’t offer automatic braking for stationary objects directly to the rear.
The M5 has a standard blind spot warning system that uses sensors to alert the driver to objects in the vehicle’s blind spots where the side view mirrors don’t reveal them and moves the vehicle back into its lane. A system to reveal vehicles in the S4’s blind spot costs extra.
To help make backing out of a parking space safer, the M5 has standard Cross Traffic Warning, helping the driver avoid collisions. Audi charges extra for Rear Cross-Traffic Assist on the S4.
The M5’s driver alert monitor detects an inattentive driver then sounds a warning and suggests a break. According to the NHTSA, drivers who fall asleep cause about 100,000 crashes and 1500 deaths a year. The S4 doesn’t offer a driver alert monitor.
Both the M5 and the S4 have standard driver and passenger frontal airbags, front side-impact airbags, driver and front passenger knee airbags, side-impact head airbags, front seatbelt pretensioners, four-wheel antilock brakes, all wheel drive, traction control, electronic stability systems to prevent skidding, crash mitigating brakes, post-collision automatic braking systems, daytime running lights, lane departure warning systems and rearview cameras.
The BMW M5 weighs 1510 to 1650 pounds more than the Audi S4. The NHTSA advises that heavier cars are much safer in collisions than their significantly lighter counterparts.