I like your idea for the 4 feet of removable plastic at the base for extra ventilation in the summer, unkadan. We have 2 hoop houses in our garden now, that are for gardening, as opposed to chicken-keeping. The whole hoop-house coop idea evolved after we quarantined some purchased pullets in the hoop house this winter (with no predator protection, but it worked out). It was so much warmer than our current coop, and the chickens seemed much more active out in the warmer, brighter hoop house, that it got us thinking about a more permanent option.
Our hoop houses are 20'x24', constructed with chain-link fence top rail. The purlins are the same material, attached to every hoop. The skids are schedule 80 PVC conduit, 3". We used 45 degree sweeps as ski tips. While there are ventilation windows up high, and double doors on both ends, it was so hot in there this summer that we had to roll back much of the end plastic (which is single layer).
I did shovel along the edges and brush snow off with a broom this winter, but we had very little snow here this year. We did learn that we should probably stake them down for the winter. We figured they would freeze to the ground, but it was such a weird winter and we had one hoop house move quite a distance during a warm, windy storm. Luckily, the hoop house with the chickens in it did not travel!
I like the idea of a moveable coop that will benefit the garden, so I'm hoping whatever we design will work out. I'm going to attempt to post a not-very-informative photo of the hoop houses, but I'm not sure I can pull it off using an IPad.
The photo isn't working. You can see it by going to my profile page and looking at the album labeled "default, 5 photos". It is photo number 5, along with some entertaining winter laundry photos should you be interested.
I would love to hear more about your hoop coop project as it evolves!