Now I do not know how 4U handles this, but VO physically blocks 5 of the 105 passenger seats in their F100 by installing non-movable tables over the middle (E) seats in the business class section, making it legal to fly the plane with 2 cabin crew only.
Seeing that the problem is 6 seats at 4U, it would be rather simple to just block off the rearmost row on their 319 (either by physical means or maybe by declaring it crew rest seats and remove them from the booking/checkin systems).