What I tried to explain is when I tested my Wi-Fi it was perfect every time I tested it. But their was clearly an issue when trying to play very large mkv files. And it was only resolved when I hard wired. So what I was trying to explain to you was even though your Wi-Fi test is showing solid Wi-Fi speeds you may still be having an issue.Do you get 4k instantly? If so, can you show a gif or small video demonstrating that it is flawless. Actually, if wifi speed tests show more than 300mbps everytime, than that means the wifi is not struggling. It means the wifi is robust. Speed test typically measure throughput pretty well, especially if you test it in different increments. Moreover, the lack of issues in apple tv and netflix with 4k hdr content proves my point. A 100mbps hardwired network won't be nearly as good as my robust wifi. The modem in my 2016 sony z9d set(150mbps +) is playing 4k instantly in prime video. The fire tv stick starts at 720p.
As far as “instant” 4k...I don’t know but I have not seen anything that would make me check it. In my theater I would notice (100” diagonal screen with Optoma UHZ65 4k HDR projector), or in my family room I would notice (LG OLED65B7A). Unless I can see something I won’t worry about it....😀
And BTW, even 100mbs Ethernet is more than enough for streaming 4k HDR/DV.