New Patreon Tier and Early Access Content available. If you would like to support AVForums, we now have a new Patreon Tier which gives you access to selected news, reviews and articles before they are available to the public. Read more.
Not a simple question to answer. Obviously a lower resolution source has to be upscaled to a full HD or 4k screen. Some televisions make a better job than others at doing this. A 720 HD movie as recorded source would be a bit unusual, although there is a lot broadcast in this format.
SD, as a worst case scenario, can look a bit rough on a 4k screen, but generally most TVs can do a surprisingly good job with a standard DVD. What you would find acceptable is down to so many variables. How big the screen is and how close you want to be to it. If you are wanting a big screen, anything over 32", then you will have little choice as most sets will be 4k anyway. If you want less up -scaling then a 1080 screen might be better, but you would need to be the judge.
The result will never be as good as a genuine 4k source, and they vary quite a bit!