A quad coupé is a car body style featuring bi-parting side doors – located either on one or both sides of the vehicle.
The bi-parting doors typically feature a longer front door and a short rear door, without a pillar in between. The configuration offers both a more 'sporty' aesthetic than compared to a traditional sedan, and the utility of 4-doors, though the doors operate inter-subdependently. This means that the forward door must always be opened before the rear-door (which opens in a rear-hinged fashion) can be opened.
References
Quad coupé Wikipedia(Text) CC BY-SA