Recently Unreal Engine 4 become free with Royalty fees. What about the code? Is it opensource or no?
From their website
Full Source Code Access
With C++ source code for all of Unreal Engine 4, you can customize and extend Unreal Editor tools and Unreal Engine subsystems, including physics, audio, online, animation, rendering as well as Slate UI. With complete control over engine and gameplay code, you get everything so you can build anything.
This is not the same as open source however, and since you pay royalties instead of an upfront price it is unlikely that there is an open source license available.