support for Metal is part of UE4 4.15 as stated in the release notes. It does require changing the Metal Shader Version for UE4 to target to v1.2 in the Mac section of your project settings as the shaders must be compiled with Metal features only available in macOS 10.12 Sierra. This makes the Metal Shader Model 5 platform incompatible with Mac OS X 10.11 El Capitan, which must then use Metal Shader Model 4.