The people at Redshark explain the difference between 4K and UHD and the reasons they refer to both as "4K" despite the small differences in resolution and aspect ratio.
"Most consumers probably don’t realise that there are two different resolutions that could reasonably be described as 4K," they write. "There’s 3840 x 2160, which we’re supposed to call “UHD”, and there’s 4096 x 2160, which is the resolution described by the DCI standard for digital cinema. DCI 4K is slightly wider than UHD, but not much. But it’s different enough that if you try to show UHD 4K on a DCI 4K screen you’ll get a vertical black bar at each side of the display, and if you scale DCI 4K to fit on a UHD 4K screen, you’ll have black bars at the top and bottom.
"We (RedShark, that is) have always described both cinematic DCI 4K (4096 x 2160) and UHD (3840 x 2160) as 4K. We do this for two reasons. First, they are very close. You would have to be very pedantic to say that you can see a difference in quality between them - especially when some people still say you can’t notice the difference between HD and 4K! Second, because we are not alone in this. Most TV manufacturers - and camera makers too - describe UHD as 4K. We just see them as two types of 4K."
The piece parses the similarities and differences further and sums up saying the term "'UHD' which we think was never the most helpful way to describe a 4K television."