My Experience...I shoot both 3 fletch 5 in on my 55#bow...But 4-3in fletch on my 42# bow...

The only downfall is with skinny shafts, helical isn't fun. The shorter feathers are easier to put helical on skinny arrows...

4x3=12 in steering  
3x4=12 in steering
------------------- the real dividing line
4x4=16 in steering
3x5=15 in steering
--------------------
4x5=20 in steering I think this is only needed if
                  the arrow isn't tuned well...

It is easier to just grab and go with 4x, but to be honest, it has never really ever made a difference to me....

I really just like the way they look.  I see no difference in flight to my naked eye.

Anybody ever tried 4x3"???  How about 4x2.5"?  I saw some of those a while back and wasn't too keen on the look but never got a chance to see how they flew.

BS

I just don't like the look of the 4 fletch sorry. As Bjorn said it's a preference thing.

I have been shooting 3 x 3.5" feathers on the skinny carbons for a few years now.  I use them for hunting and have no issue with BH flight.  I do spend a lot of time tuning and this seems to work real well for me.
ALDO

I set up a dozen arrows with 4 - 3 inch fletch,  they flew perfectly.  4 - 3 inch fletch have the same effect as 3-4 inch fletch.  Both sets have the same surface areas, so the same arrow control.  If you pluck or have other form issues you will see it in arrow flight with the shorter fletch.  Then again if your form is great and your arrows tuned properly you don't need any fletching at all.