Oh, well, if its gonna get stickied....

Second image is for Fortune on Doomshaper. The crit rates between Fate Bound and Fortune are due to the fact that Fate Bound can reroll sucessful hits while Fortune cannot. This translates to high crit rates with Fate Bound if you absolutely need the crit.
Thus, the reason you are more likely to crit on a 4 than a 3 with Fortune is that you crit with the same dice rolls, but miss if you roll a 2,1. So that roll would get rerolled if you need a 4 where it would not get rerolled if you need a 3. It was interesting to account for.