DWV vs Schedule 40

My downspouts flow into drain that moves rainwater away from the house. I'd like to extend that drain further out. Existing pipe is Schedule40. There shouldn't be any loads going over the new section.

1. Would DWV be appropriate? I had been advised to use Schedule40 but cost is much higher.
2. How do you determine the thickness that is necessary? Should I keep at 4" or could I decrease it to say 2" or 3"? Not sure how to conceptualize this yet.