at first, a "OpenGL Not Supported" message box appears and then the Editor instantly crashes after the "OK" button has been pressed.
I tried again, but this time, passing in the "-opengl" flag. it got farther – but then showed something along the lines of "OpenGL on Mac is going to be deprecated" – it was too quick to get a screen capture of it.
then again, the editor crashed: with the following output (on command line):
compile UE4 Editor for OSX via XCode
run UE4 Editor
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-38190 in the post.