I found this question from pre-release:
I am wondering if we can share code we’ve written in our project (not Unreal Source) on a public github repository now that the engine is officially released?
You are certainly permitted to share original source code (i.e., not modified or unmodified UE4 source) on a public github repository.
Does the source include the UE plugin folder? Also, do you have any FAQ (or other) reference for your answer?
You’re not permitted to share publicly any source provided by Epic (other than snippets up to 30 lines in length for discussion purposes only).
The closest FAQ entry is:
The only parts of the Unreal Engine you can’t release to the general public are the source code and tools or modifications to them; these components may only be distributed to other licensees with access to the same version of the Unreal Engine.
Great thanks!