I am by no means an authority on the trad stuff, but have been shooting bows for awhile.
4 fletch is almost certain to give the best possibe stability-properly matched of course. I would speculate that if you try 4, you should also shoot a 4 blade head.
If the arrow is properly tuned though, as others have already mentioned, fletching really shouldn't make a huge difference.
Look at the wheelies-the rage for the past 4-5 yrs. has been 3 blazer vanes fletched straight, with no helical whatsoever.. of course the speeds they are pushing with a well tuned arrow, you probably wouldnt even NEED fletching.
Again- this is where you get to into an area where returns may diminish slightly for an improvement or more importantly a percieved improvement.
I would experiment with different height/length/shape before I would consider going to an extra fletch on the shaft. Personally.
God bless,
Farmer