Card Rulings

Flamewake Phoenix's last ability will check at the start of your beginning of combat step to see if you control a creature with power 4 or greater. If you don't, the ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you don't control a creature with power 4 or greater at that time, the ability won't resolve. You won't get a chance to pay  , and Flamewake Phoenix won't return to the battlefield.

If Flamewake Phoenix can't attack for any reason (such as being tapped), then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.