I have had both just cruising the 4" is quieter but really bark when you crack the throttle also has a really deep sound i personally like the 4" better but everybody has their own opinion. Listen to both see which you like better

 

Am I wrong, aren't the Cats located in the head pipe somewhere or down by the collector and not in the pipes themselves??

 

I have had both also. I like the 4". Like RHillbilly said not as loud when cruising at highway speeds. Actually after a long day on the road the 3 1/2's used to wear on me a little. But still can't beat the sound of Rineharts.

 

I had 3 1/2s on my 07 road king.. True Duals. I put the 4s on my 12 SG true duals. I can tell you the 4s are a little deeper in sound. I like them better. Good luck in what ever you choose.