wifi AC is already more than enough for all your 4K streaming need, unless your home wifi is choppy then it's another story

LAN is a gigabit Ethernet IINM

Theoretically for WIFI streaming , it can allow speed up to 1Gbps ++ . But the CPU of the TV (which is kind of weak ) also play a part in processing the data , 100 ~ 400 mb/sec is a more realistic expectation.
Base on many of the TV models I have inspected ,  none of the current TVs ( except a certain China brands such as TCL ) has a gigabit Ethernet port.
Including the high end QLEDs and OLEDs.  It is pretty safe to assume most of them has a 10/100 , 60 mb / sec is pretty much the reasonable expectation.
For modern day Apps streaming , that should be sufficient. Primary because streaming videos suffer a major loss of video quality ( bitrates ) in the process of transcoding . 
For eg , the 4k movie on Netflix is of much smaller size than its 4k bluray counterpart . In reality , the so called " 4k " streaming video quality is more or less the same as what you see from a 1080p bluray disc , which is quite a bit of downgrade .
If you want to stream very high quality video files such as 4k Bluray Rips and remux from your HTPC / NAS , the Ethernet LAN port is unable to handle the massive data transfer . You have to rely on WIFI streaming but this can be problematic due to many of the 2018 basic 4k TVs do not support 5GHz WIFI.

Your alternative , is to connect the TV to an external storage via USB which can be very hassling .