Since Mavericks aka MacOS X 10.9.1 Apple has supported what they called 'Retina' displays on the Mac. These are screens that have a resolution that is dots per inch high enough that the individual dots are almost indistinguishable like the screen on iPhones and iPads. Smaller 4k monitors therefore can provide a high enough dots per inch resolution to qualify as a retina display.
Apple MacBooks and later iMacs came with built-in screens that qualified as retina displays and were supported by macOS for this feature. Some external monitors are also high enough resolution. However whether a monitor is automatically treated as a 'retina' display is controlled by a hidden setting in the monitor definition file. The Mac operating system comes with some displays already predefined as 'Retina' displays but in theory you can add your own custom definitions, these are stored in /System/Library/Displays/Contents/Resources/Overrides
There are also various tools for doing this for you such as SwitchResX.
There is also this website - https://comsysto.github.io/Display-Override-PropertyList-File-Parser-and-Generat or-with-HiDPI-Support-For-Scaled-Resolut…
Now whilst a 27" 4K display would (barely) qualify as a retina display as it has a dpi resolution of 163.18 a 70" 4K TV only has a resolution of 62.94 dpi and would therefore not be treated as a retina display.
See also https://www.tekrevue.com/tip/custom-resolution-mac-osx/
Moving on to how Apple implements the software side of their retina aka HiDPI support…
Apple first generate an off-screen image with the text and other vector based items e.g. lines at double the actual resolution, this also applies to icons provided at higher resolutions. These are then scaled down in a ratio you set in the Displays preferences. (The default being 2 to 1). For other elements on the screen e.g. a photo or a video these are done at a 1 to 1 ratio so they do not get shrunk they are displayed in the original size, so an NTSC video at 720 × 486 and a HDTV at 1920x1080. This to some extent depends on the application you are running supporting Apple's routines for drawing to the screen as an example Apple's own Safari of course immediately supported this so that text in a webpage benefited from this capability but Google Chrome initially did not.
See - The Software Side of Retina: Making it All Work - The next-gen MacBook Pro with Retina Display Review for a better background than I can detail here.
So to summarise, a suitable high resolution display could be used in a way that allows sharper text to be scaled without distorting photos and video, however your very large TV does not count as what would normally be considered suitable for use as a retina display. Despite strictly speaking your TV not being suitable you could create a custom setting so it does behave as a retina display.
