That said, it is of course up to debate and opinion on how to define "rules light." I look at it from a perspective of RPGs in general, not from a perspective of D&D editions. For instance, I wouldn't even call Fate Core rules light (though I would call Fate Accelerate edition.)
At release, do you think D&D Next will qualify as rules lite?
Let's take a crack at that:
Rules-heavy: The PCs must use tables or refer to rulebooks during roleplaying, uncertain actions, and combat.
Rules-medium: The PCs must use tables or refer to rulebooks during some uncertain actions and combat.
Rules-light: The PCs must refer to a rulebook only during combat.
If 4E qualifies as rules light, then I think I save to disbelieve your scale
I'd suggest bumping the scale up by 1 somewhere in the middle so you have a "heavy" below "very heavy".
I like the concept of "Everything I need to know fits on an index card" - I'd suggest the stage after it is on a single sheet of paper. That would put 4E in whatever category is after that.
Grid rules, opportunity Attacks,
conditions, a myriad of tracking terms, number of possible modifiers. Fiddly to super fiddly powers. It all adds up to make 4E quite complicated in practice even if it _could_ be very simple in theory. If you ignored a lot of stuff and attempted to streamline things.