I have not done such a test, but this post speaks to something I've noticed too. I've worked with several 4-tracks over the years, and I've always though the quality was great. Most recently, it was a Tascam 246, but before that, I got good results with a 414 mk ii and a 424 mk iii as well.
I've always wondered, quite frankly, how so many people could make 4-tracks sound so bad! I mean ... filled with hiss and tinny and/or really muffled. I'm guessing it's just people that don't know what they're doing? The sad thing is that those machines have gotten a terrible reputation because of it.
Of course, maybe you have better ears than I do, because I always used the dbx on my units (mainly because it said to in the manual), and I never noticed any artifacts or dropouts.
But here's a recent recording I made with the 246. It's not finished yet -- it's just lacking vocals -- but this is already with two external bounces (to and from the computer). So this is a two-track submix now that's on tracks 1 and 2, and the vocals will go on tracks 1 and 2.
To me, this sounds as "clean" as any digital recording I hear on this place (or cleaner, to be honest), yet it still sounds warm and pleasant (to me, anyway).
I think these machines are awesome if you know what you're doing.