I have always shot 3 5" fletches. I tryed 4" once but because I use 2 blade broadheads I had a lot more arrows plaining which really pi$$ed me off[:@]. Especially when your hunting
. I asked my local pro and he said to change to longer fletches and I've had no trouble since. i think its because the extra inch of twist is enough to stabalize the arrows flight.
As for using 4 fletches, I've always used 3 and it works great for me. As well as the fact that with an extra fletch it increases the drag on your arrow (may only be a fraction but all the little things add up) therefore slowing your arrow down.
So I reckon if 3 fletches is working for you, don't fix what ain't broken!!
Chatchya round da ridges 121553, Stalkin Steve (well probably not cause I'm in Australia)