Yeah, there is a few guys on here with 4" exhausts on their GTRs. Im using a compliance small body cat at the moment (2.5" piping) so need to upgrade pretty desperately.
I'm thinking of going with BATMBL's big body cat but a real performance brand cat might be able to flow better.
Magic Performance are supposed to have these, the number is 02 9939 1644. I should just call it but i've been busy today