Just stick with your 4" pipe and take out the cat and replace the muffler with a free flowing one, or just straight pipe the whole truck. But either way, no need, in my opinion, to switch to a 5" exhaust. Guys have made over 700 HP with 4" systems. S & B is priced fairly, have never had a product of their's, but their prices seem fine. An intake is an intake in my opinion, as long as it's not stock, intakes are intakes. If your thinking programmer, guys will tell you go with the SMARTY. I have never ran it, but I'm hearing good things. I've have good luck with Bullydog Triple Dog, but depending on the year of your truck, the Bullydog may not be the right route. Guys with 04. 5's have had good luck with them, but I've heard of some 03's and early 04's that have not liked the Bullydog too much. Again, in my opinion, try to stay away from Banks products, overpriced. These are just my opinions backed up by what I have run and had good luck with, and with what I have seen others run and have good luck with. Hope this helps.
~Chris~