Simple to do. River City 3 Railers has a three section 6' x 11' modular layout w/ tubular track that we have been using for a few years now. A 4' x 8' (to be portable) you're looking at two pieces. You may want to consider scrunching it to 47"x 95". That way you can top it with a single 4' x 8' sheet of plywood. Use a Trim Bit in your router to flush-cut the plywood to the frame. Works like a champ.
Pick a track. Decide on 1-3 loops. Design a track plan. Have at-it.... 027,031,036 and 042 curves all fit the bill. Were it me, I'd be looking at a couple of loops of FasTrack. One 031 and one 036.
Google "FasTrack 4x8 layouts". There's a bunch of good stuff out there....
Our modules are framed with 1" x 4"s with 1" x 3" cross braces w/ 3/32" Luan tops. By using smaller cross-braces, the wiring is easily tucked up inside the module(s). The tops are attached with screws and Liquid nails panel adhesive. Frames are fabricated with Kreg pocket screws and Gorilla glue. This makes for very light, yet strong modules.
Legs are 2x2's with fabricated sockets in the corners. Budget wise, you're looking at about $500 to build this.
