I don't want to burst your bubble, or shall we say burst your corrugated piping, but 4" is underdone for 8 downspouts, unless there are 8 of them on a 500 square foot house.
This was the first thing that came to mind after I read your post, and before I read the rest of the postings.
Think of it in simple terms. If each downspout was only 2x2, and you multiplied that times 8 of them, you'd need to provide 32 square inches of pipe at the end of the run, or about a 10" radius pipe, which I think is still too small. Yes, at the beginning of the run, you can get away with less, but it doesn't sound like you gave this enough thought in planning. Stepping the size after each new downspout would have been appropriate.
I'll read the rest of the postings, and come back with any other ideas I might have for you.