Not quite in full effect just yet but out of the blue it appears that Vimeo is slowly implementing 4K streaming capability. It is a feature which has been hinted at and many have been waiting for. Philip Bloom and James Miller break it down.
The trend towards 4K is undeniable and last month Vimeo announced that it was becoming a 4K capable platform. According to Vimeo themselves this is what we heard:
The key there being when they enable 4K playback. Nobody knew quite when that would be.
Then suddenly a few days ago James Miller who was shooting some test footage with the Atomos 4K recorder and Sony a7s reported that his uploaded footage was now streaming in 4K! After some testing between himself and Philip Bloom, they feel confident in reporting that Vimeo now supports 4K streaming, but it isn't without some hoops for the time being. This is what they have to say:

At the moment we don't know when full adoption and support will begin, as well as whether this will be a premium option.
via [Philip Bloom]