The power of your attacking creature is checked only when MacCready’s first ability triggers. Once it triggers, the creature will gain skulk until end of turn when the ability resolves, regardless of whether or not its power is still 2 or less at that time.

Similarly, the power of the creature attacking you is checked only when MacCready’s second ability triggers. Once it triggers, the creature’s controller will lose 2 life and you’ll gain 2 life when the ability resolves, regardless of whether or not its power is still 4 or greater at that time or whether or not it’s still on the battlefield.