You don't say how long your arrow is, but assuming you shoot a 30 incher- You can expect to drop about 15 grains in arrow weight. You can expect to pick up about 3 fps in arrow speed. You can expect your FOC to increase from around 9.4% to 10.8% (calculated, obviously). You can expect some decrease in long range (60 yards +) performance.

If you're having good results with a 4" vane though, there is no reason to go to a 5" feather. A set of 4" feathers will stabilize your arrow better than 4" vanes, will make your arrow 18 grains lighter and increase your FOC to around 11.1%, without sacrificing long range performance.

So, you won't expect much difference but, with feather fletch, your arrows will be slightly lighter, slightly faster, significantly more stable and easier to tune.