I manage construction for a living. My plumber recommended that I go with the sch 40 because he could get it much cheaper than sch 20. I bought 40' of 4" sch 40 pipe, 6 4" wyes and 10 45's for $102. It also saved me a trip to the BORG. Remembering your trials and tribulation with 6", I decided to get whatever I could buy for the best price.
Now, hopefully, someone else will be able to share in my good deal!!!