It sounds like something has disrupted their bond. Sometimes this happens with bunnies that were together from birth but maybe haven’t gone through the formal bonding process. Have there been any changes in the household, such as new pets or people? I would check them very thoroughly for injuries to see if anything worse has happened without you noticing.

Your best bet is going to be to work on rebonding them. You’ll want to separate them when you can’t supervise, and then work with them to a neutral space while supervising them. If the fighting hasn’t been too serious, they will probably sort things out pretty quickly and go back to being bonded. Be sure to prevent fighting and stop any bad fur pulling (nipping is OK though).

Also just as an aside, 4′ x 5′ is not really enough exercise space, but it sounds like they also get to roam around in a larger area daily, so that’s good. Ideally you want them to have access to 50 sq ft minimum for several hours a day so they can get up to full running speed. Maybe while you are working on bonding them again, one rabbit can be in the hutch and the other in the 4 x5 area? Then you can alternate who’s in which spot daily.