I currently have a 4" catback MBRP installer series on my truck. It bolted up to the factory downpipe. I dont have a cat on my downpipe so its a strait piece of 3.5 pipe from the factory. Would I notice any gains or difference if I purhased the 4" front pipe and bolted it to my existing exhaust system?

not a whole lot, more or less just sound.

I dont care to make it any louder just wondering about performance. According to a muffler shop here in town your exhaust system is no better than the smallest diameter pipe on it. So if the front pipe is 3.5 the entire system is no better than that flow wise.

You may notice a little sound and performance gain, but not much.

well think of it this way, it's under $100 bucks and your not gonna lose any performance really, so what the hell right. you can always sell your stock one or that one if you wanted, i'm sure somebody woudl scoop it up.