In case anyone's worried about 8K or even 10K ... The HDMI Formum has you covered.
http://www.theverge.com/circuitbreaker/2017/1/6/14189066/hdmi-2-1-specification-8k-10k-vrr
http://www.theverge.com/circuitbreaker/2017/1/6/14189066/hdmi-2-1-specification-8k-10k-vrr