Depends on several things.

-Type and weight of broadheads (Some require mor fletch to stabilize, and faster).
-Weight of loaded shaft.
-How close the shaft-spine is to being correct for your draw-weight.
-Finger or mechanical release.
-Bow draw-weight.
-Etc....

Some shooters can get good flight with 4, some need 5. Just about have to test-shoot your setup to know for sure. If you like the 4" vane but vanes do not do the job, try a 4" feather. If neither do the job, then you apparently will need 5" .

The correct vane or fletch size/type/angle set is critical. They are either right for your setup. or they are not.