I am not certain but I think that a 4 inch feather is less affected by wind and creates less drag which in turn creates a somewhat faster, quieter arrow. A 5 inch feather however creates a more stable arrow. I don't think either way it's enough of a difference to matter. I think it's just a preference thing. Again, I'm not certain on this.
As far as the offset, the helical fletch does create more spin on the arrow. As far as left or right it doesn't matter, it's just a preference thing as well. Realy the offset isn't needed, a lot of guys shoot straight fletch. Just be sure that you're using the proper fletching jig. I.E. right wing jig with right wing fletch and left wing jig with left wing fletch and don't mix offsets on the same arrow.