my group of 4 have an L shaped shed which is approx 45 square feet of floor space, and are let onto a patio area for a couple hours a day depending on the weather (not today!) which measures about 5x15 - they have a little tykes playing frame thingy to run in and out of the holes at the bottom, and a plastic chair in the shed and a chair on the patio to climb on, which they love doing, and they also like to have a big cardboard box to shred in the shed, with hay in - they like digging in the corners of the box and ripping holes in it to make a 'warren' - and they also like to eat and poop as a group, so they have 2 guinea pig cage bases side by side for litter trays - 3 rabbits can fit in one tray side by side.
the 3x6 hutch you are describing is as big as the rabbit run I have, and I can't imagine 4 bunnies in that - really they need a large territory to avoid fights and accidents (when I had 5 in the shed one bumped into another during play time when they were running around like mad things, and a serious injury was sustained)