Schedule 20 pipe or actually SDR35 thin wall pipe, is designed for sub grade or continuously supported and is stamped right on it- for use below grade only- that said: it is extremely brittle in cold temperatures and cracks very easily- these cracks occur laterally and can go undetected.
Go to a plumbing wholesaler and purchase sch 40 or 4” 636 vent pipe-I have cut out dozens of thin wall systems and will continue to flag them- repeated offences, could lead to certification suspension.

In Canada, if you have a hard time finding schedule 40 PVC pipe, you can also use ABS pipe which can be found in any hardware store and which is considered to have schedule 40 wall thickness as a replacement... The issue with the schedule 20 pipe is that it is very fragile when handling it in the winter time (develops hair line cracks length wise sometimes when mishandled in the back of the truck or onsite) and also does not have UV protection so if the pipe is exposed to the sun, it eventually gets very brittle..... the manufacturer does not recommend its use above ground for that reason.

Good to hear the rationale for the schedule 40. I've only been using it indoors, with a schedule 40 muffler at the exhaust point. Thank you for the feedback as I search out a supplier...

Schedule 20 does exist in theory the manufactures could make it in 4" they just don't have a market for it. The US Mitigators saw SDR 35 as an inexpensive alternative in highly competitive markets without realizing that it was not rated by manufacturer for above ground use. I would not want to be in those shoes in a highly litigious society that they work in.

In addition to competitive market, the earliest radon work was done by folks whose building experience was in states with no statewide building codes. So it took a while for some of these issues like pipe schedule and garage penetrations to get into practice.

Interesting that you mention garage practices, in Canada I have been corrected many times by designers and code officials that a fire collar is not required by our building code as a garage wall is not considered a fire seperation bbut is considered a smoke seperation (we still install fire collars).