Unless you've got an internet connection that absolutely tears, you won't be able to get a great stream, but you can swing on over to YouTube to see some samples.  To enable the 4K resolution, click the button that would normally let you scale up to 1080p and pick "original resolution."  Then wait a while and hope your computer can handle it.

One thing can definitely be said for this move though, YouTube is thinking ahead.  Once 4K televisions become the standard, people are going to want 4K content.  Other streaming services are slow to adapt, so you can be sure that companies like Netflix won't be enabling it.  YouTube, on the other hand, is right on board.