I have the 4" cone just on a PVC steup of my own and works fine...
Are you just planning on running an open cone? Or is this for a certain CAI?
If this is just for an open cone than you can choose either one, you just have to make sure to match up the fittings. You can get any of that stuff at Menards. If it is for a specific CAI then look up the specs on it and they will tell you what size cone comes with them.
If you are just replacing your filter, then measure the inlet and buy that one
-Rich