Hi, I use 4" SDR 35 glued, not gasketed for about 30ft. of head and 400 ft. of penstock. All of it is above ground, in the streambed. Going on 4 years. I have had no problems. We painted the pipe for UV protection. It is surprisingly flexible, but resilient. I've worked with SDR 35 a lot and much prefer glue to gasket. The gasket can be very hard to seat. I have had issues with shut off valves at the generator, the moment you shut it off all the weight of the water above hammers the length of penstock and I think the fittings are stressed. I've not have one fail though. Good luck,
David