UE4 is under a proprietary license where you can’t distribute your fork. It’s source is available, but it’s not “open source” in the copyleft/freedom sense that

UE4 is under a proprietary license where you can’t distribute your fork. It’s source is available, but it’s not “open source” in the copyleft/freedom sense that the phrase is usually used in. As for closed-source competitors: game engines have always provided their source under proprietary licenses to people who paid enough, so the only real difference now is that you can get access to it (but with limited rights) without paying anything. But besides a few big names (Unity, Unreal and CryEngine/Lumberyard) there are quite a few closed-source competitors in use that don’t have generally available source code, although many of these are used only within a publishers development studios (idTech usually got open sourced after some time but until then the code wasn’t available to the public, Game Maker Studio, Frostbite etc)