when sett brawler enters play as a 4 might unit, and gets might reduced (by stupefy for example) before his buff. at the end of turn, when stupefy expires, does sett player trigger fiora legend ability

Yes. When Stupefy expires at end of turn, Sett's Might goes from 4 to 5 (base 4 + his permanent buff), and that transition from below 5 to 5+ is exactly when a unit "becomes Mighty" (Rule 709), triggering Fiora, Grand Duelist.

This is the same interaction as any temporary -Might effect wearing off: the "this turn" effect is removed during expiration, Sett's Might re-evaluates to 5, and Fiora's trigger fires and goes on the chain. Players can even respond to that trigger before the next phase — effects played in response just won't be cleansed until the next end of turn (FAQ #742, #358).

Note: Sett's play buff is a permanent counter and doesn't expire, so it's Stupefy dropping — not the buff running out — that makes him go 4 → 5 at cleanup.