SDR is a replacement installation waiting to happen. In other words it is "junk". Anything thinner would be absolutely worthless even though you would use SDR fittings with it. The big "difference" between SDR and sch40. is that sch 40 will not collapse when it is buried. SDR WILL collapse, crack, flatten out, etc. from the weight of the dirt., and you should NOT use it inside a building for a drainage system. Only use it where you can get to it with a backhoe to dig it up. THe other factor is that the I.D. of the pipes SHOULD be equal for interconnection purposes which is why SDR has a 4" i.d. but a smaller o.d. because of the thinner wall. "Thin wall" pipe would create a ridge at EVERY Fitting because its i.d. will be greater than 4".