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Either one works fine I prefer 6s only because they don't offer 7.5s. killed a pile at 40 with the old 3-2-7.5 federals wish they still made that shell or better yet a longbeard with 7.5s. Will 4s penetrate more sure but my goodness a turkeys head is not that tough. They aren't that tough to kill a #8 low brass game load with 1 1/8 of shot will pulverize one at 30 yards I know from experience tried it just to see. But out of your 2 choices the 6s are the way to go.
Your pattern fails before penetration, assuming head/neck shots on wild turkey gobblers.