Changing how long the neck pocket is won't change how far the pickups are from the bridge - because it doesn't look like he's planning on changing the neck, just where the join is. The neck pickup will always start something like 1/2" from the 22nd fret.
From what I understand it's about half the thickness of the body and half the fact that the cutaways start on both sides about a half inch past the end of the neck pickup cavity.
Done right, there shouldn't be any structural problems with either pcoket length - I personally like te 4" better. You've gotten about even votes on both sides, so pick the one YOU like the best!
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im going with the 4"
thanks, adam