4ft is pretty small. What kind of car do you have? If you can get a couple flat rails 8' plus that'd be a good start. Rails arent that heavy if you use 8th inch ish and don't skirt it. Also pvc is light enough even with the 2x4s.
2 flat rails are pretty versatile. 16' drop rail, 16' afrwme, 16, ebow, 16 aframe, etc. Coulw also build a 2 or 3 foot section as a nub to add into stuff for donkeya, mini battle feels. Or a longer one and could go z tube etc.
Not sure what youre trying to do, where youre setting them and how much time you have.
Also sometimes framed out rails with no skirting are wobbly even after they set up and a fresh set theyll be rocking. Good feet would be really helpful.
If feet get in the way if throwing it in or on the car maybe drill a hole in the bottom of the leg, and in a wide foot and throw the foot on woth a pin when youre setting it. Could be quick and give you more stability without having to have any extra weight on the sides. Wouod prolly have to weld a little t to rail legt jist above where the foot will sit so it doesnt rock. I feel like it could work.
Idk. I'm just thinking. Ive had raiks as big as 22 on my moms mini van growing up though. Friggin lawlessness.
Good luck, post pics and stuff. Love ghetto builds.