Answer: You Will Need a CAT6 Ethernet Cable
It’s possible to use the older, cheaper little brother to CAT6 – Cat5E, but I wouldn’t recommend it. Uncompressed 4K video requires a whopping 12 Gbits of data per second. That is astronomical. Fortunately, modern-day compression techniques can whittle down that number an order of magnitude. Cat6 cable is only a few dollars more than Cat5e and it offers quite a bit more shielding and anti-crosstalk features. You’ll also be future-proofing your home theatre setup (or wherever you plan on watching a 4k video) so that you’ll be good when you decide to start watching an 8K video.
One word of caution though, the cable is only a small part of the equation when it comes to data speeds. The other hardware like your modem, router, switches, and ethernet cards, all need to be upgraded to Gbit or higher