The media-streamer arms race continues unabated, with Roku taking the wraps off its Roku 4 less than six months after shipping its retooled Roku 3 . The Roku 4 joins the ranks of the TiVo Bolt and Amazon's Fire TV in delivering 4K resolution and supporting 802.11ac Wi-Fi, and a new version of the Roku OS that improves features introduced with the Roku 3. The new model looks distinctively different than previous generations, with a thinner and wider form factor designed to help cool its higher-performing components, including a quad-core CPU and the aforementioned 802.11ac Wi-Fi adapter.