As for wasting your money on a 4K set vs. a full HD set, it almost isn't even a choice anymore. The "budget" 4K recommendation at RTINGS is about $500.

Also, although it is mentioned in the text, keep in mind that not all content is high-bitrate 4K:

Cable TV? Probably compressed 1080p or 720p, depending on the channel. If it's Comcast/Xfinity, they may even squeeze three channels into the bandwidth of two.

Streaming? Compressed 1080p, with the occasional 4K.

Old (but not that old) Blu-rays? 1080p.

If you're sitting in your leather club chair, Oppo UHD Blu-ray remote in one hand and a glass of '92 Screaming Eagle Cabernet in the other, then sure--slide your chair into the sweet spot at the optimal distance per these charts.

Don't get me wrong: when I replace my ten year-old Sony, it's going to be with the biggest LG CX or C1 OLED my wife lets me get. Just realize that trigonometry isn't the whole story.