Wow, I didn't expect that rude comment but it is no big deal.
Thanks for the clarification Lynn but it is still confusing to novice me and evidently other GTO owners. I answered based on old research when I was first looking into trading in the old GTP for a GTO. But I decided to pass on that for obvious reasons which I won't discuss in this thread. Here is an article, a GM press release that covered the exhaust configuration for a 2004 Pontiac GTO/Holden Monaro.
Click here
Now according to my buddy at the local Pontiac dealer the first CATs in each bank are full catalytic convertors monitored by the downstream 02 sensor(s). Cost is expensive at $830 each. The ones you see in between the forward CATs and the mufflers are quasi CAT convertors and a combination resonator built into one but really just classified as resonators. Hope that makes sense.
Backpressure is low at 40kpa but there is room for improvement. From what I understand GTO owners are doing the resonator delete with no side effects except to sound. See the following fairly recent thread at ls1 forums.
Click here
Edit Update:
Here's another thread that answers the four CAT convertor mystery.
Click here