I've been meaning to reply to this thread, but something always seems to distract me, then I forget. Anyhow I think your on the right track though. The 4 inch ADS irrigation tubing would allow more root space for the pea plants than the downspouts or rain gutters would. Pea plants will grow quite a big root mass. Here's a link to the pea plants I grew in that exact ADS tubing a while back: Third system
, I also attached a picture of the root mass after harvest (I guess I never added it to the thread). The ADS tubing also has a couple of advantages over the PVC tubing other than being cheaper (including end caps). First it's black inside, and that blocks light, yet it's white outside and that reflects light (so it's ready to go). Second is it's much lighter, and easier to cut than PVC. Just remember that the ADS tubing is not PVC, and even though PVC glue works intentionally, the ADS irrigation tubing takes it's own glue.
Also, another hint is when I constructed my system I forgot it would need to be opened to get the root mass out to reuse it. That's why I had to cut it in half to do so, then silicone a coupler in the center that never really worked well. Because it wasn't glued in, the weight from the water made it flex to much in the center. That made keeping the water level correct difficult. I forget who came up with the idea of using a rubber band as a gasket (like a large "O" ring). By placing it around the tubing before putting the end caps on. Thus tight fitting, yet removable, as well as still water tight. I haven't tried it because I haven't rebuilt the system after the second use (the tubing is still in the backyard), but I would test/try it out for sure. If the rubber band binds up when slipping the end caps on, try putting a thin layer of Vegetable oil on it first.