You are correct, to a point. Their website does say the MK4's have the Index Matched Lens System, but it is in the variable models. The fixed models have the MultiCoat 4 Lens System, along with the MK2's.
By the way, I received my replacement scope in from Valdada yesterday. They said they didn't have any 6-24's so they sent me a SFP 3-18.
As far as the dual spring erector system being on the MK4, they advertise it for the VX-L and VX7 but not the MK4. I wonder why? If this is the toughest most durable system they have why don't they put it in their toughest most durable scopes? And if they do why not advertise it? Maybe I'll call them back and ask.
So it would seem that the variable MK4's are just VX-III's with tactical knobs and etched glass reticles (if they even have that-not advertised).
Leupold has diluted their charactor with about a hundred too many hodge-podged models... None identified by revision, or even year... So when you buy a 'new' Leupold scope, you have no idea what your really getting. Neither does the seller..
In the past few years I've concluded that even Leupold cannot keep up with it..
That said, the MK4s are my choice among ANY scope available, at any price.
Just hope they will not dilute the MK4 line like the others.
I already seen they now have a MK4 'PR', which is about as offensive as a 'Mustang-II'.