Once the last ability of Temur Ascendancy has triggered, lowering the power of the creature won't stop you from drawing a card.

If Temur Ascendancy leaves the battlefield, creatures you control lose haste. If they haven't been under your control since the turn began and don't otherwise have haste, they can't attack this turn. If they've already attacked, they remain attacking creatures.

If a creature is entering the battlefield under your control, consider static abilities to determine whether its power is 4 or greater. Spells, activated abilities, and triggered abilities can't be used to raise the creature's power in time to have Temur Ascendancy's last ability trigger or to lower it in time to stop it from triggering.