well the 4 mill is going to overall add more cc's
your port job you get for the motor will determain where your power is really made at aswell as the pipes that complement the port job .
You like to run 4th,5th,6th on ure banshee now ? defintly go with some cpi's and a aggressive style woods or dune port . Bigger carbs would be nice but not too big , you still want to keep the bottom end so i think 30mm's would be great . +4 timing ... Lets not forget the good reeds to complement ure setup.
You really are building a hole new motor once you start porting and changing with the parts to increase hp....
it will open your eyes man , really !!!
Any banshee nut will agree with porting...... MORE FLOW , MORE GO ....