It's an unofficial grade system lol. I think AquaTerra talked a bit about it in another thread I read. Basically, "quads" are supposed to be the top tier or primo grade.
Take the same exact strain and some people will grow it better than others. Quads, or AAAA, has the smell, bag appeal, taste, and high. Mids or AA-AAA, usually is lacking in some department. As AquaTerra pointed out, some of the commercial quads are being pumped with PGR to give some strains better appearance and yield. Better to grow your own smoke. But if you want to sample strains you never tried before, it is pretty wide open in Canada now with mail order.
I will tell you though, I've smoked buds from some MOM's labeled as quads that I personally would only label it as AA+. One example was "Sophies Breath" I got from a MOM labeled quads. It had the bag appeal as far as appearance but it lacked in smell and taste.