Can I do wuyishan, longyan or yongding(the hakka village) and xiamen in 4 days and a half?

Yes, you can. And you are suggested to take trains to move around.

As I know, the bullet train from Wuyishan East to Longyan leaves at 12:23 and takes about 4h 40 minutes, CNY370.5 for a first class seat, CNY236.5 for a second one.

Longyan to Yongding: the trains leave at 19:58 and 21:12, which takes about 1 hour and CNY11 for a hard seat.

Yongding to Xiamen: the trains leave at 4:40 and 5:11, which takes about 3h 30 minutes, and CNY96.5 for a hard sleeper, CNY131.5 for a soft one.