Are you talking about for permanent set up (“home base”), or exercise space? And do you have 1 bunny or two?
A 4 x 4 pen OK for 2 bonded bunnies for a home base but it is not big enough if they spend 100% of their time there (the HRS guidelines are 8 sq ft the cage, and 24 sq ft for exercise). Buns should get at least a few hours of exercise per day in a larger space, and honestly more than 24 sq ft is better if you can. You just want the bun to be able to get up to full running speed if they want to. Usually a bunny-proofed room is plenty, and it’s good to time exercise for when they are most active (morning and/or evening).
Where are the pens located? Would it be possible to bunny proof the room and give them a couple hours of supervised roam time in that room each day? Or to attach a second pen for exercise time so they can get a bit more space for zoomies?
All of that said, we do the best we can, and sometimes temporary situations are not perfect. It’s possible to make smaller improvements over time. When I lived in a smaller place, I built my buns a condo with multiple levels so they could have more sq ft and some interest, so that’s an option too.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.