You are absolutely free to release your game’s source code (and content) online! We love the idea and are doing this ourselves with the new Unreal Tournament game. You can put the whole project up on a public repository if you want.
Just beware if you are using content or code that came from the Marketplace, which might not be allowed to be redistributed in “raw” form. Epic’s free learning sample content are usually okay to use in your open projects however. If you end up modifying the engine/tools source code, remember that you are only allowed to distribute UE4 engine code to other active UE4 subscribers (usually with your private fork on .) Releasing code to plugins you’ve created is totally fine though.
You are allowed to release the code for your games even without an active Unreal Engine subscription. It is your code. 
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