Hello,
Incase what's been said was confusing. 4k is the cinema standard, while UHD is the consumer standard. 4k is also not just a small difference in resolution, what is required to take advantage of the new UHD displays is HDMI 2.0a, which will be able to feed HDR (High Dynamic Range), among other specs, which in laymans terms means better picture quality than anything ever seen before, in the consumer market.
This is where the new UHD displays shines, over the older FHD TV sets, and this is what will be used to market these. Resolution by itself doesn't mean a whole lot, much like you look closely at your arm, you see small detail you otherwise won't see further away from you. We could easily use Apple's definition of this, which in short is just called Retina Density. You're actually going to be able to see a change over older UHD sets, which is very exciting to say the least.
Like mentioned above, it upscales, as there's no consumer UHD content available, that actually looks good. And by that I mean that FHD Blu-rays still beat the different UHD streamers.
Before we can determine what's wrong with what you're seeing, first we're going to need a lot more information. However, as a starting point, always change the default aspect ratio to not overscan, 99% of the times TV's ship with a factory setting that's set to 16:9. This means that it's overscanning, and you need to use Screen Fit, or Just Scan, or Dot by Dot, TV manufacturers name this differently, but you should make sure it's not overscanning. Even a 5% overscan, is enough to completely get rid of small detail, worst is that's it's going to soften the whole image. Incase I explained it poorly, for the best image without profssionally calibrating:
Picture Preset: Movie/Cinema/Theater
Sharpness: 0 (1 if 0 creates artifacts)
All "Dynamic" or other weird options that should make no sense to you: OFF
This will ensure best picture quality possible, again, without calibrating it, which is expensive.
All the best!