how early can I take public bus from Zhangjiajie city to Wulingyuan Scenic Area?

I will arrive at the train station at 4 am September Sunday and want to take bus to Wilinggyuan scenic area as early as possible to avoid huge line up for Bailong Elevator. Do you know when the first bus depart from bus station? Wulinggyuan entrance opens at 6:30-700am. Thank you in advance.