What is the energy of a 4 kg apple that is sitting on a 2m high tree branch?

Potential energy always to a reference altitude, which in this case will be ASSUMED to be at the ground level.
Also, it is assumed not to be moving with respect to the ground.

Then kinetic energy is zero.
Potential energy = mgh
where
m=4 kg
g=9.81 m/s ²
h=2 m

So 
total potential energy
= mgh
= 4 kg * 9.81 m/s ² * 2m
= 78.48 joules (with reference to ground level)