If there is no run access, 4 sq ft per chicken is pretty tight, at the very least you should expect your hens to get bare-backed, hopefully it would stop there. If you have runs going out from both sides of the building, and could subdivide *them* too so that each breeding pen has a run, then no problemo.
The general concept of removeable dividers is sound -- I use that a lot in my own coop. I have a long row (6-7' wide, the length of the building) on either side of a center aisle, and I have removeable partitions that I can insert where I want them. In the case of the E side of the building, it is a floor-to-ceiling divider of chickenwire stapled to 2x2s. Along the W side of the building, the "pens" are actually 4x6' chainlink dog pens from when a previous owner of the property had the building set up as a kennel; I've removed the bottom half of the dividers between adjacent pens but left the top half, so my removable dividers only have to be half-height.
Because of the structure of my building (and I'm not sure that anyone else *except* me is using a converted dog breeding/boarding kennel for their chickens, so it is probably not widely applicable
) it is super easy to put the partitions in and out, and I don't need to worry about doors b/c a whole row of doors is already built in there, all along the aisle.
However, for what you describe, you will have to put some thought into how to engineer things so that the doors (in particular, although the rest of the structure too) are strong and rigid and durable enough while still having the panels easily set up and removed. My best suggestion would be to *permanently install* two 4x4 posts into that pen, located at the junction of two adjacent doors for the breeding pens. This will give you hard structure that your divider and (more importantly) door panels can be attached to. It will be much more stable, and last a lot longer without getting wibbly, and be much easier to construct, than if you were trying to make it all free-standing. I can't see that the posts should be a problem when the dividers are not installed, you and the chickens will just have to walk around them is all
You may well find that you need to make the bottom 3' or so of the pen dividers solid (I just staple on old opened-out plastic feedbags
) so that adjacent roosters don't get crabby and belligerent with each other... although it depends on the temperament of your particular roos.
Good luck, have fun,
Pat