If you use PVC schedule 40, be aware that there are at least two kinds of 4" PVC schedule 40. When I built my house 25 years ago, you just went to the home store and got the PVC. Now someone has decided we needed a lighter product and is making a foam filled 4" PVC schedule 40. Unfortunately, it will turn to an oval if you stand on it where the original product would hold its shape. You may have to go to a plumbing supply to get the good stuff. I got the bad stuff at Lowes. I buried 100' of the 4" schedule 40 foam filled PVC under a heavy traffic load area before I figured this out.