I've been testing this myself on the same bow. same exact arrows,same helical, only difference is I'm testing 3" vs. 4" feathers.

My Conclusion: 4" have a noticeably better steering effect on anything but a perfect shot (form-wise)..even field points.. yet at longer ranges (35 plus), the difference in drop is also very noticeable in the case of 4". They impact a couple inches lower.