Quad bikes are also known as ATVs. ATV is short for all-terrain vehicle. While they are a distant cousin of the pedal bike, for the most part they have little to nothing in common with them. The first difference is that they are motorized. The second is the number of wheels seems to have doubled.
From there things get truly odd. Quad bikes have mechanically assisted braking, and some even come with power steering. They’re not licensable for use on roads, but they see extensive use off road, on back country trails. They are also used on farms; because their smaller size can go places a full sized truck could not.
Quad bikes originally came out as three wheelers, rather than the four wheels they sport now. However the three wheel chassis was unstable; and it caused some severe accidents, including fatalities. This created a compelling reason to shift to a more stable platform, and the quad was born. Today if you buy an ATV, it will be one with four wheels, not three.