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It doesn't matter whether a player has 4 or less life than they started the turn with. If a player lost 4 life but also gained 6 life, that player still lost 4 life.
Knight of the Ebon Legion looks at your entire turn to determine whether a player lost 4 or more life. It doesn't matter how many players lost 4 or more life, only whether anyone did. It also doesn't matter whether Knight of the Ebon Legion was on the battlefield when they lost life.
A player loses life if they pay life.
A player doesn't need to have lost 4 life all at once. If a player loses 2 life twice during a turn, that player lost 4 life during that turn, and Knight of the Ebon Legion receives a +1/+1 counter.
