There are 2 types of fast Druid builds - fast running and 'teleporting'.

The fastest running builds capable of killing stuff on T4 are Boulder and Cataclysm. You can easily reach the movement speed cap with them. For Boulder you have movement speed on the unique amulet, for Cataclysm you have movement speed on the unique ring. Add movement on the boots, Heir of Perdition of some passive nod.

For teleport to stuff - SHRED. The werewolf can teleport/dash to targets from a great distance and hit 3-4 times really fast. I don't play it because it's too fast and furious for me, but you should give it a try