I want to share the resolution to both my wifi and 4k problems I had with a Raspberry Pi 4 and Libreelec 9 and 10 that turned out to be on my side.
1. 4k 59.94hz and 4k 60hz didn't appear in the video mode list
The highest refresh rate I saw for my TV on the video mode list was 30hz but the TV does support 60hz at 4k. I know that there's suppose to be support for 4k-60 in the latest version but I read it was disabled recently. Either way I was able to finally get a signal by adding "hdmi_enable_4kp60=1" to the config.txt. Then another problem appeared. Green thin lines in random places on the image.
That was caused by a fake 4k-60 cable I got on Amazon, I checked the cable jacket and the cable is only rated to 1080p-60, I imagine I was getting 4k-30 because I was lucky but 60 introduced artifacts caused by freq bandwidth limits.
2. Wifi 5ghz weirdness (Bad key message)
Using both the 9.x and 10.x versions I was never able to get a connection to my 5ghz network. It would take a long time during authentication and then say "Bad Key". Once that happened if I were to try again the entire system would crash and restart. I thought for so long it was because I was using a DFS channel (100 with a 80mhz width) with my router.
One day I just randomly decided to switch my USB power adapter from a Samsung 2.0amp@5v to a Targus 45watt USB-C which offers 3amp@5v .
Now I am able to connect to my 5ghz with zero stability issues.
I know the pi4 recommends a 3amp 5v adapter but since I was able to run everything besides 5ghz wifi perfectly, and that includes previous overclocking on a raspberry pi OS install with overvoltage at 6 and the cpu at 1750, I would have never suspected power especially on just one band.
Maybe this helps some people 
Love 10.x btw!