I believe in the technical talk forums there was a thread started about PBR in general. Lots of good info in there to migrate over to the official PBR thread we start. Here's the link
I pulled a bunch of the posts here into a PBR thread: http://www.polycount.com/forum/showthread.php?t=124683&page=2 edit: also I've merged in that PBR thread from tech talk as well, thanks artquest.
There's basically two options for textures when working with PBR, linear and sRGB. Take a look at this page http://www.marmoset.co/toolbag/learn/pbr-practice , you'll notice each chart will list a sRBG value and a linear value. It's part of working with a linear workflow, there's a lot of confusing concepts when working in…
With correctly implemented PBR, materials should look the same across all game engines. Game engines are always going to have their own artifacts and characteristics, but the fresnel/specular/reflections are much better in UE4.
The workflow isn't too different from UDK. The main difference is in how you create textures for PBR, but importing assets, dealing with lightmaps, setting up materials, etc, is pretty much exactly like UDK.
This image has the PBR values you'll want for leaves http://www.marmoset.co/wp-content/uploads/materialref01.png And you'll want to use subsurface for the translucency https://docs.unrealengine.com/latest/INT/Resources/ContentExamples/MaterialNodes/1_7/index.html
Is there any news on being able to modify the lighting model in the material editor? PBR is great and all but it still has that 'Unreal sheen' that I disliked so much from UDK (aka everything looks like plastic and goddammit I need a fresnel).