6owed,
You'll have heXX getting a 4 inch down pipe in some trucks most manufacturers use a 3.5 inch down pipe to a 4.0 system size. Whatever you do be sure to get a system like the MBRP or AFE that allows you to bolt the cat in totally un modified. Keeps the dealers happier, and MBRP and AFE have pipes that allow you to unbolt the cat and run it straight too as an option too. Give us a call if you have any questions etc.
Mark @ DPPI