4K input is supported, but limited to 30Hz (HDMI 1.4 with HDCP 2.2, as far as I know). Same as the S (which was confirmed to pass through 4K input signals).
HDMI :: Manufacturer :: HDMI 1.4 :: FAQ for HDMI 1.4
I don't think there are any 4K@60Hz broadcasts anyways, most boxes (at least from the only two 4K TV services here in Canada) only have the older HDMI hardware, and broadcast signals are at 24 or 30Hz.
I agree that it's not really future-proof, but the reality is that there's never been a huge amount of content at 60Hz, even before 4K. Games are really the only type of content where you'd notice the difference.
Even spatial sound only requires HDMI 1.4. The original Xbox One supports spatial sound output (Dolby Atmos available, DTS:X confirmed for future update) through its HDMI 1.4 Out port. Not sure about input, I don't have any devices that output a signal to test.