Within UE4 support for and usage of shaders is gated within the Shader Model 5 feature set. This also requires support for Compute and either increased sampler units or separate sampler/texture binding amongst other things, that OpenGL 4.1 does not provide. Consequently we can’t currently support SM5 on Mac OS X, hence why there’s been no support for on the Mac, even though Apple’s OpenGL claims to support it.
I have previously tried to decouple from other aspects of Shader Model 5 in order to test Apple’s OpenGL support, discovering several bugs in some of the GPU drivers provided by OS X. As far as I am aware there is at least one issue still remaining that prevents compatibility with UE4’s support.