If Sett - Brawler spends his buff to get +4M, does that trigger Fiora's leader ability?

Yes, spending the buff DOES trigger Fiora's Leader ability. Here's why: When Sett activates his ability "Spend my buff: Give me +4 [M] this turn", the buff is spent (removed) as a cost during step 4 of activating abilities. This causes Sett to drop from 5 Might to 4 Might (losing the +1 from the buff). Then, when the ability resolves, Sett gains +4 Might, going from 4 to 8 Might. Since he crosses from below 5 Might to 5 or greater Might, he "becomes Mighty", which triggers Fiora's "When one of your units becomes Mighty" Leader ability. Note: Fiora also triggers when Sett is first played, as his Play Effect buff causes him to go from 4 Might to 5 Might, making him become Mighty for the first time.