The Z4’s pre-crash front seatbelts will tighten automatically in the event the vehicle detects an impending crash, improving protection against injury significantly. The 4C doesn’t offer pre-crash pretensioners.
The BMW Z4 has standard driver and front passenger side knee airbags mounted low on the dashboard. These airbags helps prevent the driver and front passenger from sliding under their seatbelts or the main frontal airbags; this keeps them better positioned during a collision for maximum protection. Knee airbags also help keep the legs from striking the dashboard, preventing knee and leg injuries in the case of a serious frontal collision. The 4C doesn’t offer a front passenger side knee airbag.
The Z4 has standard City Collision Mitigation, which use forward mounted sensors to warn the driver of a possible collision ahead. If the driver doesn’t react and the system determines a collision is imminent, it automatically applies the brakes at full-force in order to reduce the force of the crash or avoid it altogether. The 4C doesn't offer collision warning or crash mitigation brakes.
The Z4 has standard Post-Crash Braking, which automatically apply the brakes in the event of a crash to help prevent secondary collisions and prevent further injuries. The 4C doesn’t offer a post collision braking system: in the event of a collision that triggers the airbags, more collisions are possible without the protection of airbags that may have already deployed.
The Z4’s optional lane departure warning system alerts a temporarily inattentive driver when the vehicle begins to leave its lane. The 4C doesn’t offer a lane departure warning system.
The Z4 offers optional Park Distance Control to help warn the driver about vehicles, pedestrians or other obstacles behind or in front of their vehicle. The 4C doesn’t offer a front parking aid.
The Z4’s optional blind spot warning system 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. The 4C doesn’t offer a system to reveal objects in the driver’s blind spots.
To help make backing out of a parking space safer, the Z4’s optional cross-path warning system uses sensors in the rear bumper to alert the driver to vehicles approaching from the side, helping the driver avoid collisions. The 4C doesn’t offer a cross-path warning system.
The Z4’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 4C doesn’t offer a driver alert monitor.
The Z4 has standard BMW Assist, which uses a global positioning satellite (GPS) receiver and a cellular system to remotely unlock your doors if you lock your keys in, help track down your vehicle if it’s stolen or send emergency personnel to the scene if any airbags deploy. The 4C doesn’t offer a GPS response system, so if you’re involved in an accident and you’re incapacitated help may not come as quickly.
Both the Z4 and the 4C have standard driver and passenger frontal airbags, front side-impact airbags, driver knee airbags, front seatbelt pretensioners, four-wheel antilock brakes, traction control, electronic stability systems to prevent skidding, daytime running lights and rearview cameras.
The BMW Z4 weighs 800 to 970 pounds more than the Alfa Romeo 4C. The NHTSA advises that heavier cars are much safer in collisions than their significantly lighter counterparts. Crosswinds also affect lighter cars more.

