Netflix is now streaming 4K content for eligible PC users in addition to their usual services. To watch content in 4K you'll need a new machine or 2-in-1 device that can run 4K, which also needs to be running Windows 10 too.
Also, you'll need at least a 7th generation Intel Kaby Lake processor and of course a 4K display to take advantage. Additionally you need to upgrade to the monthly premium subscription, costing £8.99 a month, which includes Ultra HD streaming. If you happen to have all that, then enjoy some 4K content via Windows 10.
Microsoft is also taking advantage of the Windows 10 exclusivity to help market their Edge browser. Do you meet the requirements to stream in 4K?