In Oregon we recently re-legalized 4-plexes and some other, denser forms of housing by right in our cities. But it's a pretty incrementalist reform, and it'll take time to see the effects.
The quadplexes have to age and trickledown to being boring housing stock; around here the apartments and quads are going for about the same as older stand-alones and only a bit cheaper than new stand-alones. And when you factor in the usual HOA cost the payment ends up being the same.