I have a Dell XPS 8920 with an I7/7700 (Kaby Lake), an RX480 card running on a Cox ethernet connection with a 60 mbps download rate and an LG3259-B 4K monitor and could not get either Netflix or Hulu to play anything even even after running Windows (10 Home) Update, Dell Update and the AMD updater. Playback started and then stalled although not completely but nearly so. The installers had updated my AMD driver to Crimson ReLive Edition 17.7.2.
After locating and manually installing Crimson Relive Edition 17.10.1 I can now stream video from both Hulu and Netflix without problem, and can confirm 4K streaming for Netflix (I haven't tested Hulu 4K yet). Note that not much 4K content is available at Netflix with the standard $99 Prime account but there is enough to test it.
It seems that the problem was the AMD driver. Hope this helps others with the same problem.