The effect fletching has on an arrow is based on drag created by both physical dimension and or weight. So essentially 3 ea 3" fletch is 9" overall, so if you use the same fletch for a fourth fletch, you now have 12" and additional weight that creates more drag AND lessens FOC. Also If you run 3ea 4", you can run 4ea 3" of the same design and notice little or no difference. FOC change for the better means less/lighter fletch or heavier upfront broadhead/insert, etc. Ive seen tons of guys go from 3ea 3" to 4ea 2" not noticing that their arrows don't recover and fly as well, and mostly because it is hard to tell if you haven't explored the concept. And yes, we have tested this to come to conclusion because this isn't my/our personal findings, just ours determined like results.