You are correct I have never owned a Mk. 4 (though I have owned several Leupolds), but I have shot one on many occasions on a good friend of mine's rifle. I did not think that the glass was terribly great
Since it's not your Mark 4, I bet you've never checked to see how clean the lenses are. Maybe your friend hasn't checked either. I keep my objective and ocular lenses very clean and always check them before a match. You would not believe the difference it can make if there's a layer of dust on the lenses. The muzzle blast kicks up all kinds of dust so if you shoot prone you really need to keep an eye on the objective lens in particular. If you have a sunshade, it's easy to forget about cleaning.
A few months ago I shot a 458 SOCOM with a Super Sniper on it. It was the single worst scope I've ever looked through but there could have been a tub of Vaseline smeared all over the objective lens for all I know ... it wasn't mine so I didn't inspect it. I could now rant about how Super Snipers are rubbish, but that wouldn't exactly be scientific would it.
As I keep saying, once you get over a certain price point, glass "quality" (which is TOTALLY subjective by the way with no empirical data ANYWHERE to be seen) is moot. Most folks don't compare apples to apples anyway. The size of the exit pupil can have a HUGE affect on how you perceive the scope to be in terms of clarity and brightness but that may have nothing to do with the quality of the glass, or how your target looks at 600 yards, or how your scope handles mirage, bright sun etc. There are numerous aspects to a scope (already mentioned) and unless you own one and use it on a regular basis, which includes inspecting/cleaning the lenses, you really can't consider yourself to be an authority.
This thread has got my feathers up but I offer no apology. It's about time the record was set straight. From here on out, if someone wants to slam Leupold without one shred of evidence or based solely on what they've read, then I'm going to call them on it. I've had a long and enjoyable relationship with Leupold products and will absolutely accept and welcome well thought out, fact-based criticisms of their products, but unsubstantiated, anecdotal BS just ain't gonna cut it. THR should be better than that.
On a side note, I spent some time yesterday ranging "targets" with my Swarovski Laser Guide range finder. Swarovski is well known for their quality in optics but I can't say for sure that the glass is better/worse/same as the Mark 4s that I have. The view through the Swarovski is excellent and everything looks crystal clear, but it does through my Mark 4s too ... everything looks sharp through my Mark 4 spotting scope as well.