Defender (This creature can't attack.)
As long as you control a creature with power 4 or greater, Drowsing Tyrannodon can attack as though it didn't have defender.
If Drowsing Tyrannodon's power is raised to 4 or greater, it will allow itself to attack.
Once Drowsing Tyrannodon has attacked, it will remain an attacking creature even if you no longer control a creature with power 4 or greater.