Random thoughts...
For the size modules you're talking about, I don't see any real need for any kind of frame. Lately I've been building dioramas and layouts on sandwiches of Gatorboard (I'm just starting my fourth). Gatorboard will not warp, even when wet. Two half-inch thick sheets laminated together make a mighty rigid and very lightweight layout base. At four feet long, you would have no need to provide center support, even with fairly dense scenery. For a finished appearance, I attach 1/4-inch foamcore to the edges, cut to follow the scenery elevation countour, and apply vinyl woodgrain. You can get Gatorboard in black, which also looks good by itself.
I think I might recommend adjustable shelving brackets. That way you can easily set the layout to whatever height you want, and change it at will.
My WR&N layout was in a 10 x 11 room and was supported on seven shelf brackets, three on the short wall and four on the long. They were the "super bracket" variety that can support hundreds of pounds. I set them at shoulder height, and that left a great deal of space in the room for other activities (in my case, a workshop and an art studio).