Sadly we have heard from Roku that HBO NOW will not offer a 4K option at this time. So if you had plans to buy a Roku 4 with the hopes of seeing HBO NOW in 4K you will have to wait.
HBO has officially posted that they “continue to investigate 4K video support, but it is not available at this time.” So there is still hope but as of now HBO has no plans to offer 4K Ultra HD on the Fire TV or Roku 4. It should be expected that at some point HBO will offer 4K support but the question is will it be this year or five years from now. The only people who know are at HBO.
As sales of 4K TVs continue to increase as the price drops you should expect more services to start releasing content in 4K. They also listen to their subscribers, so if you are an HBO subscriber and want 4K support make sure to reach out to HBO and ask them to bring 4K to your favorite streaming device.