4" is typically plenty, especially shooting feathers. I am actually shooting mecjanicals with 3" feathers and a right helical and getting excellent groups. Not sure I could say the same if I were shooting fixed blade, but for mechanicals my 3" seems to be doing fine.

Another benefit especially with feathers is the less feather you shoot, the less noise they will make. The 3" ones are deadly quiet on the way to the target. I have some fletched with 5" and they have some hiss on the way to the target.

BTW< I am shooting a 29" long arrow and the 3" is working good for me. I'm not so sure arrow length makes a huge difference if your bow is tuned well.