Are those 4 bags full (as in weight)? I bet 4 full (as in packed by average joe with bricks) bags would bust that bin after a month. Not to mention 9 seats across (3, 3, 3) needs 9 bags so 8/side plus other junk is still going to cause bags to be gate checked.
Until our ever dumbing engineers or greedy bean counters get it into their heads that they need to take the area of an average fat American at 5 foot 10 inches - 220lbs (seated) with a roller bag and laptop/carry bag then multiply that space by the number of seats the aircraft is designed for we will continue to have problems (and sh!tty designs) and piss off the flying public.
Maybe years ago you could get away with designing seating/bags for average 80% or less capacity but today with capacity discipline and bag fees - every plane is full and every pax is lugging a roller and carry on bag down the jetway.