Separate two different things. 4K and High Dynamic Range, ie HDR10, Dolby Vision and HLG. Two newer ones looking to join in, are HDR10+ and just announced IMAX mode.
For you to see a discernible improvement over 1080p you need to be within approx 1.5-2 times the distance of the Diagonal. So for a 60", 5 foot screen you need to around 90", 7.5 feet. 120", 10 feet away. Most people have the TV further away than this, probably tucked in a corner, so unless you like sitting closer, 4K will not achieve much for you. The same rules apply for projectors.
High Dynamic Range on the other hand is easily visible at normal viewing distances. There is no requirement that HDR has to be restricted to 4K, however this is the Norm, so one will usually be accompanied by the other. "Good" HDR requires over 1000 nit peak brightness range, actually closer to 4000. Most Oleds are around 400. Doesn't not mean it will not work or that 400 nits of pure white is not totally annoying in darkened room. Try watching Mindhunters on Netflix and have you retinas burned with each new city.
400 is good enough for most people with controlled lighting, 500-600 is now more common in the latest Oleds. Samsung and other LED manufacturers usually get around 1000 or more which makes it more suitable for normal living room conditions.
Best picture currently, if the ambient light can be controlled, is an OLed. Oleds will however struggle in bright rooms. Try not to waste the beauty on Game Consoles. Burn in, like Plasma, can occur but you really need to work at. Two year on and hundreds if not thousands of hours, my Oled has never had this problem.
Best for normal living rooms is something like Samsung's QLed or other similar Led technologies. Local zone dimming helps improve the blacks for Leds, higher number of zones are better than lower. You can still get a degree halo issues with local zone dimming, however this less noticeable in brighter rooms.
For me UHD discs are currently overpriced. I will take a good bluray over these any day and a lot of the time Dvds are fine to. To show off your 4k Tv there are a good number of shows on Netflix and Amazon, other services are available. Some are just 4K, others come with Dolby Vision or HDR 10. Samsung TVs do not I think support Dolby Vision since they do not wish to pay the license fees, but others do. HLG (Hybrid Log Gamma) is largely the preserve of Over The Air Broadcasters. The BBC in the UK is trialing this.
8K will be worse for viewing distances, probably nearer 1 to 1 and it is at least 4 to 5 years away. Hdmi 2.1 whilst it will support 8K, is more about higher refresh rates for 4K gamers currently. eARC will be a nice addition once all the gear is in place.
