By The Numbers
The basic rule of picture clarity is that the higher the pixel count, the higher the image's resolution. 4K image technology was initially introduced for digital cinema, which standardized the spec for digital projection in movie theaters at 4,096 x 2,160 pixels. A 4K TV comes equipped with this same resolution—roughly four times the resolution of the previous 1080 standard—whereas UHD offers a display resolution of 3,840 x 2,160. While this is slightly short of "true" 4K, UHD resolution is often rounded-up and referred to as 4K for the sake of simplicity. But in reality, there is technically more texture and detail shown in a 4,096 x 2,160 display resolution than UHD.
Truth be known, buying a 4K TV versus a UHD TV will not make much of a difference to the average viewer, with both offering excellent resolution. However, if having the highest possible picture clarity is essential to you, you'll want to look for a 4K unit that displays at 4,096 x 2,160.
There is already a newer version of UHD boasting an even higher resolution of 7,680 x 4,320, commonly known as 8K. While 8K is the (eventual) future of high-resolution technology, these models currently come with a hefty price tag and a shortage of available 8K content. So, fear not, as it will be a while before they become more commonplace and affordable.