Still no source on https://github.com/EpicGames as of now though.
"Through GitHub you can access UE4 C++ source code from Epic and also see code contributions from other developers across the community. We have lots in store for this channel, and we're looking for feedback on what you'd like to see here as well."
How do I submit Unreal Engine 4 changes back to Epic?
GitHub is our channel through which the community can fork and modify Epic's code alongside our own engineering team. Source code changes you check in at GitHub will be viewed by the community, and if the vetting process goes well, we'll incorporate your code into UE4. This means you'll help lots of people!