RE: Difference between 4" and 2" vanes?
The standard used to be 4" vanes for hunting setups, but the trend the last few years has been to use Blazers (2"). Of course this almost means a drop away rest is a must, and from what I've been reading on several forums there are a fair amount of people that still have rest clearance and cable clearance issues with the Blazers.
Part of the reason they have become popular is because they weigh less than many 4" vanes. Depending on what brand you use some 4" can weigh up to 35 grains for three of them whereas Blazers weigh in at 15 grains. Also I've weighed some brands and had variances of 4 grains per vane within a pack of 50----not very good quality if you ask me.
Because of the clearance issues I'm still not sold on them. My personal preference in vanes is Duravane LP 400's. They tip my powder scale at just about 19 grains for 3 of them and have a 3/8" profile. Weight tolerances between vanes tend to be very close, too, varying only a tenth of a grain. Usually I use feathers, but I always keep a couple arrows with these vanes around just in case I might (rae) decide to hunt in a rain. That and they give me something to play with.