What ever exhaust that you decide to go with, make sure that it is turbo back. There is no real benefit in going cat back. Between a 4" and 5", that entirely up to you, the 5" has no real performance gains either. I have a 4" turbo back cat delete MBRP exhaust and with the Street Race Lite tune that I am running, it sounds awesome. I kept the MBRP free flow muffler on and it is more than loud enough at WOT. Many will say to just straight pipe it, but I am glad that I didn't because after I tuned it, she is about as loud as I would want it to be. Silverline also makes a nice exhaust!
P.S. Welcome to the org! This place is a fountain of knowledge and there are many techs etc... on here that are nice enough to help us out.