Defeat Dragonoir*. Be warned: Dragonoir can cast slow, damage reflect, and buff himself in addition to his long range and knock-back attacks.
Once Dragonoir has been defeated, enter the portal on the right side to enter the 4th stage.
*Note: Occasionally, Dragonoir will not spawn. When this occurs, merely enter the portal on the right side to go to the next stage. If this occurs, your final reward experience will be lower than normal.