At the moment the only computers are the ones that have Kaby Lake generation (7th gen) Intel processors. This limits the computer selection to just a handful of new laptop models. However, rest assured that 4K on PC will be available to more people as the new chips come more popular and become available on desktop computers. Also, Netflix might make it available on other processors as well.
Right now, though, it is far easier to watch Netflix in 4K with a Chromecast Ultra or a 4K-capable TV.