If my Seth spends his buff to go to 8, does he go from 5 then 4 then 8 and I can use Fiora legend as one unit becomes mighty, or is he considered as already mighty?

Yes, your Fiora Legend ability will trigger when Sett, Brawler uses his ability to go from 4 Might to 8 Might.

According to the official rules and established FAQs for this interaction:

1. Sett starts at 5 Might (4 base + 1 buff). He is currently "Mighty" (5+ Might).
2. You activate Sett's ability: "Spend my buff: Give me +4 Might this turn."
3. Cost Payment: As part of the cost to activate the ability, you spend (remove) the buff. Sett's Might drops from 5 to 4. He is no longer "Mighty."
4. Ability Resolution: The ability resolves, granting him +4 Might. His Might increases from 4 to 8.
5. Trigger: Because his Might changed from less than 5 (4) to 5 or greater (8), he "becomes Mighty" at that moment. This triggers Fiora's Legend ability.

Relevant Rules:

• Rule 709: "A Unit 'becomes Mighty' at the moment its Might changes from being less than 5 to being 5 or greater."
• Rule 708: "A Unit 'is Mighty' as long as its Might is 5 or greater."

Because Sett ceases to be Mighty when the buff is spent, he is eligible to "become Mighty" again when the +4 Might effect resolves. This is consistent with RiftJudge FAQ #30 and #641, which confirm that spending the buff triggers Fiora's ability because Sett drops below the 5 Might threshold before regaining it.