yep, at distances that would allow an immersive experience for increasingly popular 80/84/90-inch panels, 4K is great to have and allows a virtually invisible pixel structure. Since the HDTV market has become particularly stagnant I wouldn't be surprised if content came sooner rather than later.
I haven't seen the 84" Sony yet, but I'm pretty familiar with 4K up-conversion via the JVC DLA-X70 on display at work. The 4K "e-shift" (3840x2160) looks great on our 115" screen and makes Blu-Ray content look pretty amazing.
desktop + headphone Sonus faber Toy monitor|REL T Zero|Sony UDA-1|Sony MDR-Z7|Sony MDR-1000X|Audioquest NightHawk|Ultimate Ears Triple.fi 10|Sony NW-A35
ht Sony A1E 65" OLED|Monitor Audio Silver RX6, RX Centre, RX1|SVS SB-2000|Sony STR-ZA5000ES|Sony UBP-X1000ES|Sonos Connect + Peachtree Audio DAC•iT|Panamax M-5100PM
stereo Sonus faber Toy tower|REL S/5|Teac UD-501|Sony TA-A1ES|Sony SCD-777ES|Sony ST-SA50ES|Sony 43" X800D|Sony UBP-X800|Sonos Connect|Furman Elite-15 PFi