red devil rx580 8g

Just got a 4K 120Hz TV, my card will push 120hz at 1080p & 1440p, but only 60hz @4K. TV only has HDMI inputs, and I am just trying to figure out if the 60Hz limitation is due to the cable I'm using, it's new and supposedly HDMI 2.1/8K ready, or if it's a refresh limitation on the HDMI output...