When shooting broadheads you need a little more satiability in the arrow. So the 4" will help any problems with arrow flight as the arrow leaves the bow. Remember, the fletchings job is to correct the arrows in flight and keep the arrow flying straight and true to the target.

 

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OK, as you asked for hunting situations only, there are no advantages in shooting 3" vs 4". 4" will give you the greatest stability. 3" will gives you a little more speed, but for hunting situations speed is almost a non-issue. Stability and accuracy are what count.

 

Well to me, that makes the most sense.....I was suggessted by a Big Name Archery Manufactuer to try and shoot a stiff 3" Vane...I really wondered Why should I?
But they why do I see so many setups these days using 2.5" and 3" fletchings for Hunting....
Guess they just dont Need more broadhead stablity...

 

How about 3 1/2 in. vanes?

Since there's a trend these days toward smaller "mini" broadheads, a larger fletching isn't always necessary.
This year I've tried a Flex Fletch 360 vane (3.6 in.), with good results. I'm shooting a medium sized fixed BH (RM Ti-100), and the smaller fletching seems to be stabilizing them just fine. Some of the advantages I see with this size vane is:
A. Reduced weight, improved FOC percentage
B. Less likely to be affected by wind like a 5 in. vane,
C. Less clearance problems with rest.

Personally, I fletch mine with an aggressive helical and shoot them with a drop away rest. So far... I like it.