Comcast doesn't have 4K content (yet). If you put on Netflix, it should notify you about content they have that is in 4K (i.e. Daredevil, Luke Cage, Jessica Jones). If you have an older HDMI cable, you should buy a new cable with 4K quality. You may not notice a difference now, as older HDMI cables have limited 4K capabilities and 4K isn't big on the market yet, but as it continues to grow your current HDMI cable (if older) would not be able to show you the true content of 4K. The cables can be a bit pricey, but worth it. Look for Monster Cables with 4K; should range around $60 dollars or so.