I have never ever seen a 4 deck game in IL, IN, MO, MS or NJ ! Older BJ books reference them as common.
6D appears to be vanishing from the areas I frequent, in favor of 8D.
Looking at BJA3 page 23 - time to double bankroll 8D is pretty discouraging - (even at TC1 and higher) - my gosh if there are still 4 decks being played I'd like to visit there!