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.

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 {R}, and Flamewake Phoenix won't return to the battlefield.