Since it is just 4 ft wide that would be perfect for cutting some thin plywood and drilling a few holes through it to attach to the ends of your frame or drive some rebar in the ground to hold them in place.

I suppose it depends on how you plan on accessing what's growing. If you are going to fold up the sides that you can make the ends so they stay in place. If it were mine I would be having one end having a framed in automatic opening vent. Then from the wood frame to whatever hoop material cut in greenhouse film then secure it to the wooden frame with thin wood strips and secure it to the hoops with clips like these - https://www.johnnyseeds.com/tools-supplies/greenhouse-and-tunnel-supplies/hardware-accessories/snap-clamps-for-3-4%22-pvc-or-1%22-emt-7036.0.html?gclid=EAIaIQobChMI9qaGlMnW7QIVUPHACh2vQQj6EAYYAiABEgKydfD_BwE Then for the other end just trim the greenhouse film to size and secure with the clips. I usually use bricks along the bottom edge to hold the film to the soil. I have made clips out of 3/4" irrigation line. The trick to using those is soaking them in hot water before slipping them on. Good luck