Always quad-track guitars..?? also Panning...

I always quad track. Two tracks 100% panned, two tracks 80% panned. If I was doing a different style of music besides metal I might experiment with another approach, but hands down this is the most reliable way to get great tone for metal rhythm guitars.
 
no, you're not.
(assuming it's possible for them to doubletrack....as you said, if they can doubletrack 4tracking shouldn't be impossible...just the same thing again)

There's a lot more leeway allowable between left and right than there is between two takes on one side - when I first tried quadtracking it surprised me how much of a difference that could make.

When I quad track I get a bunch of 'good' takes and keep the ones that fit well with one or more other tracks.

Jeff