Is 4K the same as Ultra HD?
In addition to 4K, you also often see the term Ultra HD come up, sometimes abbreviated to UHD. As mentioned, 4K is an umbrella term for different resolutions where the number of horizontal pixels is around 4000. In practice, 4K and Ultra HD are often used the same, but technically they are not quite the same. The term 4K was originally created by the Digital Cinema Initiatives, a collaboration of major film studios, for the 4096×2160 resolution. This served as a successor to the previous industry standard 2K, which had a resolution of 2048×1080. This version of 4K, called DCI 4K or Cinema 4K for clarity, has as many vertical pixels (2160) as the "standard 4K," but counts slightly more pixels horizontally. This results in more widescreen and a different aspect ratio, namely 17:9 instead of 16:9. Some advanced consumer cameras also feature DCI 4K, such as the Fujifilm X-T4 and the Panasonic Lumix DC-GH5S.
However, when a manufacturer gives a product a 4K stamp, it almost always means the aforementioned 'standard 4K' with the resolution 3840×2160 (marked with * in the image below). However, a better and more logical name for this resolution would be Ultra HD. But as 4K has proven to stay, manufacturers are eagerly using the term. For clarity, some brands do use the term 4K Ultra HD, where you can be sure it's 3840×2160.