Hi Nils. I believe you would benefit greatly from this surgery. You have a fairly long chin and a reduction would make you look better without a doubt. Though from the after pics I think you could even reduce a little more than 4 mm because the difference is barely noticiable, especially from the front.
I'm also considering getting a chin reduction and I'm looking for other people's opinions, especially strangers, because as you say, since friends know you from a long time they get used to your face or maybe are afraid to hurt one's feelings when giving advice.
I would be very grateful if you could do those before/after images with photos of myself, as I have no clue how to use photoshop. My email is email@example.com
. Hope to get in contact with you soon.