I hadn't either until I got a 4K TV and actually watched a program in 2160 resolution. When I loaded Spiderman in Ultra HD and only got 1080, I started to do research to see why.
In order to get 4K, you need to have make sure that all cables in the chain can pass 4K.
Many older cables (but not all) can pass 4K. But if you want to be certain, you can buy newer HDMI cables that are certified to pass 4K.
Price has nothing to do with picture or sound quality. If the cable is able to pass the resolution you want, it will look the same as any other cable that can pass the resolution you want, regardless of price.