Apart from watching 4K close-up similar to sitting in front of a PC screen, I don't think most will be able to tell much difference between 1080p and 4K unless they either have a very large (>60") TV and incredibly sharp eyesight. Even then, I'm sure most people can't even see the full 1080p resolution either of their existing TV from the sofa.
If your PC screen resolution is 1920x1080, a simple test is open up "Computer" full screen an take a screenshot. Save it as a JPEG on a USB stick, plug it in your TV and display that image. Now, sit on the sofa and see if you can read the various location/drive names down the left column or even the date/time shown at the bottom-right of the image. If not, then obviously you're not even seeing full 1080p and probably barely 720p also.
Even my friend who does wedding videos is surprised at just how many people don't realise that DVD is not even HD. He said that many of his customers specifically ask to have their wedding filmed in Full HD and when he asks if they would like it on Blu-ray disc or USB stick, the customer says "DVD is fine".