I mentioned in another post on this thread, I've been using the cardboard cores that flooring and carpeting are rolled around. I get it from free from a flooring outlet. It is mostly 4", sometimes 5".
I've even made 90 degree bends by making 2-45 degree corners with a short straight section in between, by cutting the pipe on my miter saw, and glue together with construction glue. That is probably too much work, but it does work well enough if you are careful.
Still, I am planning to replace with 6" S&D PVC eventually.