Quad mechs are a rare sight, but they do still exist (some analyst claim 1 in 50 mechs are quads, others say as few as 1 in a 100 or even less). Never popular or prolific to begin with, there are only a handful of minor quad mech production lines still in existence. 

Quads do not share their bipedal cousins’ humanoid appearance, looking more like conventional tanks walking on four legs. Capabilities are somewhere between that of a mech and a tank. They retain the mech’s superior cross-country mobility, are more stable than two-legged mechs (but harder to right if they do fall), but not quite as agile. They are usually heavily armored, but tend to carry slightly less weaponry than bipeds of the same tonnage.

There is a great deal of bias against quad pilots amongst the general mechwarrior population. Most consider them little more than tankers. Even on Solaris they aren’t exactly popular: The annual Quad Games draw comparatively fewer spectators are prize money is a joke.