A standard "Default Lit" 2-sided shader in UE 4.4
This 2nd image is the same tree using the new Foliage shader with the Sub-Surface effect. Notice how nicely lit the backs of the leaves are. This scene does also contain a skylight so there is a little indirect illumination as well, but it is not responsible for the sub-surface effect.
"Two-Sided Foliage" shader from UE 4.7
Here's a screen shot of the material setup. The Diffuse texture is also used to dictate the sub-surface illumination. Wiring it through a multiply parameter allows for some fine tuning. Epic recommends that you use perhaps a separate texture for this that has more contrast to dictate how the sub-surface effect varies within the texture map (twigs, veins, etc.). Cranking up the Multiply effect on the Sub-Surface Color can eventually wash out the effect much above 3 or 4. The Multiply parameter is not required on the Diffuse property itself but I like the added control to adjust the look.
A material using the new "Two-Sided Foliage" shader in UE 4.7
Below are also 2 screen shots "in game" of the tree in UE 4.4 and then in 4.7 Again, there was no skylight used in the UE 4.4 sample and perhaps a little too much "Emissive Color". In addition, a simplified Normal map was used, just the diffuse shader run through the nVidia filter in Photoshop. In the 4.7 sample a proper Normal map was used to specify and enhance the random orientation of the leaf normals, producing a far superior looking tree without any additional enhancement.
Tree in Unreal Engine 4.4 with standard "Default Lit" shader
Tree in Unreal Engine 4.7 using "Two-Sided Foliage" shader
with enhanced Normal map and Skylight for indirect illumination.