Buying a 4k TV is like buying a sound system that offers a range of 20Hz to 20KHz. By the time you can afford it, you can’t hear above 12 KHz.
Rule: never pay more for a sound system if you can’t hear the difference.
Rule: never pay more for a TV high definition screen if you can’t see the difference.
Unless you have your own home theater large enough for guests, you cannot see the difference that a 4k TV makes. Here is a handy chart. See for yourself.
What the chart shows is that, for a 50-inch screen, the benefits of 720p vs. 480p start to become apparent at viewing distances closer than 14.6 feet and become fully apparent at 9.8 feet. For the same screen size, the benefits of 1080p vs. 720p start to become apparent when closer than 9.8 feet and become full apparent at 6.5 feet. In my opinion, 6.5 feet is closer than most people will sit to their 50″ plasma TV (even though the THX recommended viewing distance for a 50″ screen is 5.6 ft). So, most consumers will not be able to see the full benefit of their 1080p TV.