Go with a 5 inch exhaust. It sounds a little deeper and better then a 4 inch in my opinion. Plus on all these new diesels they are all choked down with all the emission junk. The stock exhaust on all of the new diesels chokes them down. Get rid of the stock exhaust and it will help your truck a little bit. I have a 2002 f250 7.3 with a 4 inch exhaust and it sounds damn good but if it had a 5 inch it would sound a bit better to me.
19 year old angler. Work hard Fish Harder!!!