I don't have any experience with the Mark 4 mount, but I do have a Bullberry Contender carbine barrel.
I have read somewhere that Fred moved the mount holes foreward so that they were drilled over the bore instead of the chamber where the metal is thicker.
The foreward mounted scope with me put the scope in a lot better position. I have a factory carbine barrel and I have never been able to find a comfortable position, seems the scope is too far back. In my opinion moving the holes foreward is an improvement.
As far as Bullberry vs Virgin Valley, I have bought both an if I buy another custom barrel it would be a Bullberry.