no doesn’t add more detail on non4k content. If it doesn’t have it it can’t make it up. As an example a 4k picture might show a pimple on a newscaster. A 1080p picture may not. If you increase the size of the picture from 1080p to 4k it can’t then decide to add a pimple to it. Not the best reason to get 4k, to see pimples, but you get the idea.
The upscaling from 1080p to 4k can actually create a much sharper picture than just a 1080p so you may be please with the improvement with your current box.
The difference between that and 4k hdr is huge though. Mrs and I were watching a 4k hdr version of Gardians of the galaxy 2 at the weekend. The wife said to me it’s like we’re there with them! It’s that good at times. some films are better than others. 4k discs are better than streaming but the 4k hdr content on Apple and Netflix can look really special