84-inch? why not smaller? Is pixel density a problem? Sounds like four 42inch 1080p panels glued together .140cm is about the largest TV for home, anything else is for cinemas xD
EVen at that size (84 inches) you have to sit 1.65 meters from it to not lose the full 4k-experience (longer away and the pixels will blend into each other). So, for the normal 3 meters tv-vewing distance it is way to small!
The benefit is that you now will be able to watch 1080 at about 3 meters distance (or thereabout, think it was even 3.5 meters), wtihout losing any visual acuity. As it is now a 42 inch 1080p tv have same same pixel density as a 84 inch 4k tv and to fully get the benfit of 1080p you have to sit at 1.65 meters for a 42 incher (which you can enjoy normally with a 84 incher).
Problem is, that there is still a lot of lowres sd-content out there. Terestrial channels, cable, websteraming and so on will look just terrible on such a big screen.