If Sett (Brawler) has received 4 damage and uses her ability to remove the buff and get +4 attack, does the unit die?

Ruling: Yes, Sett would die. When you activate the ability, removing the buff is a cost paid to add the ability to the chain, which means Sett loses the health buff before the ability resolves.

Sequence:

• The buff is removed as a cost to activate the ability
• The ability is added to the chain
• Before the ability can resolve, cleanup happens
• Sett has 0 health (due to the 4 damage and loss of the buff) and dies during cleanup
• The ability never resolves

Nuances:

• If removing the buff were part of the effect (not the cost), Sett would survive because the +4 attack would be gained simultaneously with losing the buff during resolution