What is the exhaust system called with 4 pipes on it?

Question

I have an 07 yamaha YZF-R1, I seen some 4 pipes underneath the tail of another bike. I was wondering if that was a custom design or can i buy those pipes from somewhere

Answer 1

It's called "fourpipes"

Answer 2

I heard something similar to that. In New Zealand, we had a four into one pipe. We called it siamesed.