What is this flat pin battery connector on my quadrocopter called?
It is known by a few different names:


Walkera plug
Losi connector or Micro Losi
Micro-T or Micro-T/B/DT




You can buy the 4 pin version here.

The housings and crimp contacts are also manufactured by Molex. The part numbers are:


Female housing: 51005
Female crimp contacts: 50013
Male housing: 51006
Male crimp contacts: 50012