I am building a vacuum dust collection system and looking for 4" PVC pipe with 1/8" wall thickness. What "schedule" should I look for?

It will take a Large Vacuum, you may look into what the Banks use for piping, They don't need as large vacuum as you will.

I would go with Schedule #40 with a pipe wall of .237 inches.
SAshedule#80 at ,33 would be overkill. Good luck, Tom

I have seen some solid underground drain pipe used in a shop. Wall thickness is about1/8 and I believe schedule 40 fittings fit. Unless you are using a cyclone and commercial vacuum source, a large capacity shop vac will work.