it just depends on what you are wanting to do with ur truck. if you are just wanting to put a few mods on it then 4" will do but if u are wanting to go full out then you will probably want a 5". a 5" will probably have a deeper drone. but i have a 4" on my 7.3 and it sounds great
Over stock- you'll definately see quicker spool, and it'll run better on the upper end... Lower backpressure for the same boost. (assuming you have a newer truck with a wastegate)
Also, a few guys have made over 600 hp on a 3" downpipe (Largest that's easily fit in an OBS) ...
96 Powerstroke: 6637, downpipe to dump exhaust, 140v IDM and a shift kit.
84 Honda Sabre V65 1100: pipes, jet kit and new tires. Faster than I think I'll ever need.