What-4 was a car in the 1971 Hot Wheels series. The name of the car is a play-on-words to what is likely a first reaction thought by viewers; "Four engines? What for?!?" Early versions of this casting are prone to a special type of metal fatigue in the chassis known as "crumbling". The diecast was improperly mixed in the factory, and over time, with age, the bases oxizide, develop cracks, and warp through no fault of the owner. This was not limited to the What-4, other castings made during the same timeframe suffered from this too. They are known as "crumblers", and they do hold a small collectable value to some collectors that want them in their collection.