According to all information, this is a stand-alone game, so like you say, you dont need Uncharted 4 for this.
But, Owners of UNCHARTED 4: A Thief’s End Digital Deluxe Edition, UNCHARTED 4: A Thief’s End Triple Pack, and UNCHARTED 4: A Thief’s End Explorer’s Pack will receive UNCHARTED: The Lost Legacy as a download at launch.
Ah... I understood, thanks for clarifying