Direct streaming requires CPU power too? Sorry I am new to this
(My 4k UHD movies are mostly h.264)
Nothing is free, and something still has to decode the video and audio data and store it in a frame buffer and display it on the screen...
With a 4k image there are four times as many pixels to generate when decoding the data. Even with compression in the original data, there will still be more data to decode in the source data stream than for a 1080p file. And as mentioned, that Atom processor is very low power, even by Atom standards (it's also 8 years old). Regardless of that, you still have the problem of needing a driver for a suitable USB display adaptor - I very much doubt that exists in an easy to use form, and even if it existed, for LibreELEC you'd possibly have to build your own binary. In my view, it's just not worth it, given that there are other ready build solutions available.
As mentioned above, a Raspberry Pi 4 could easily do it. If you're only using h.264, you could also look at some other second-hand Intel solutions. For example, a Celeron N3060 can manage 4k @23.967 (and probably at 30 fps as well) with h.264, but absolutely cannot with h.265 since it lacks the hardware acceleration. Newer Intel CPUs (the Gemini Lake platform for example) are relatively affordable and can play full 4k UHD blu-ray rips without issue.