Is it legal to stream unreal engine 4 on twitch as long as you dont stream the source code?
Correct. As long as you do not “leak” the source code.
LOL
Sorry but it kind of strikes me as funny considering Epic is the biggest leaker of them all.
To put it into perspective everyone at Epic would have to be fired for breaking NDA. 
Epic is allowed to ‘showcase’ whatever they deem necessary since they are in full control over what gets posted or not.
I doubt any ‘sharing’ on their part is done without approval of their superior.