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.