text: Avatar Aang's last ability looks at the whole turn when determining whether or not Avatar Aang should transform, not just actions you've taken since he entered or came under your control. For example, say you airbend, waterbend, and earthbend, then play Avatar Aang. If you firebend later in the turn, then when Avatar Aang's ability resolves, he'll transform since you've done all four this turn.