This problem was not exactly technical. Big Content was not happy about turning its products over to people who could copy it at 4K.
Now Netflix is supporting 4K streaming through Microsoft's Edge browser, but you'll need a new PC to do it. Netflix is only supporting Kaby Lake Intel Core processors, and there aren't many laptops that support 4K display and the new Intel processors.
Microsoft is using the 4K Netflix support as a marketing effort for its Edge browser and to encourage people to upgrade their hardware. So why has Edge got the edge here? To satisfy Big Content, Netflix had to promise some rather heavy DRM to prevent streams from being captured and redistributed illegally. Hardware decryption features simply aren't available on older Intel processors. Kaby Lake chips supports 10-bit HEVC, a popular 4K video codec. Edge plays nice with those codecs.
Still it is probably better to splash out on the Chromecast Ultra or the latest Roku if you want Netflix in 4K.