4K and Ultra HD are not the same thing and it's important to know the difference

Video feeds come in a variety of shapes and sizes, and it can be confusing trying to wrap your head around each individual resolution and aspect ratio. One particularly head-scratching example is 4K Ultra HD, otherwise known as 4K UHD. As it turns out, 4K and UHD are actually two separate, distinct resolutions, even though they're often lumped together under a single unified banner.

Source : https://www.pocket-lint.com/differences-between-4k-and-uhd/