I have read something on this and it relates to the temperature of the inner 2 cylinders.
The outer cylinders cool better having no cylinders on one side of them, and 2 and 3 are located in between two other HOT cylinders, so heat transfer is limited as compared to the outer two cylinders.
A lean cylinder burns hotter than a rich cylinder so it makes sense to me.
Not too sure how much of a difference it would make....
l8r