Loving the discussion. How much more difficult will it be to stream 4K via wifi than 1080p?
Depends entirely on the bitrate of the UHD (aka 4K) material and the quality of the WiFi (and the congestion of your WiFi environment).
It is quite easy to stream UHD stuff at Netflix's low max bitrate of ~15Mbs (because they are 24-25p and pre-encoded) in a nice quiet rural environment with only one WiFI network and with the WiFi router/access point nice and close with only plasterboard walls in the way.
It is much more difficult to stream UHD stuff at a continuous live broadcast rate of ~25-30Mbs (with live encoding and 50p) in a busy urban area with 40+ WiFi networks in range, sharing frequencies, and with your router at the other end of your house or flat with thick stone or brick walls in the way of the router.