Reading up on www/press on this & copy/paste some points: the Premium HDMI Cable Certification Program is the HDMI industry's means of identifying cables that have been tested to perform at a specific level that will carry 4K content. Any cable rated as 'high speed' should pass 4K content provided it is indeed a true 'High Speed HDMI cable'. HDMI Licensing acknowledges the issue of some "high speed" cable not being up to the task of actually handling the full HDMI 2.0 bandwidth, saying: "Although many current High Speed HDMI Cables in the market will perform as originally expected (and support 18Gbps), some unanticipated technical characteristics of some compliant High Speed HDMI Cables that affect performance at higher speeds have been found. These cables are compliant with the Category 2 HDMI Cable requirements and perform successfully at 10.2Gbps, but may fail at 18 Gbps." ........ hence, HDMI Licensing introduced a third testing tier in addition to "Category 1" (Standard) and "Category 2" (High Speed): the "Premium HDMI Cable."
If a cable is failing to carry the content, its seen as "Sparkles": individual dropped-out pixels, "Line dropouts" where a whole line of video, or the rightward portion, drops out, "Intermittently flashing or jumping picture" indicating that so much picture data is being lost that the display is losing sync, & finally no picture. It does not involve blacker blacks & more verdant greens.
Finally I took my Mark Grant G1080 HDMI which is claimed (& tested) for 1080p around to my AV buddy to run on his 4K setup & it worked just fine.