Re-enactment groups often put four large fletchings on each arrow instead of the usual three, to slow them down more when they are being shot at opposing armies, to reduce the chances of any injuries. (They also have rubber tips, of course!) Some fletching jigs, such as the Tollgate, can be changed from three- to four-fletch quite easily, but if you have an old fletching jig that will do only three, you can make it do four using the following method.

Put the arrow in the jig, but don't do the cock feather, just the two shaft feathers. When the glue has dried, take the arrow out of the jig and turn it round half a turn. Put it back in the jig and you will find that the shaft feathers are now on the side where the cock feather should be, and you can now fletch two shaft feathers again. You finish up with a four-fletched arrow. Simple, innit?