Deathtouch, indestructible
Rhonas the Indomitable can't attack or block unless you control another creature with power 4 or greater.
{2}{G}: Another target creature gets +2/+0 and gains trample until end of turn.
Once Rhonas has attacked or blocked, it will remain in combat even if you no longer control another creature with power 4 or greater.
You don’t have to attack with another creature with power 4 or greater for Rhonas to be able to attack. The same is true of blocking.