RPi4 does not support 4K H264 hardware decoding so bad (CPU decoding) results are expected. It supports 4K only with HEVC.
I'd suggest reading this https://wiki.libreelec.tv/configuration/4k-hdr to ensure the whitelist and other settings are optimal. I would remove force_turbo and hdmi_enable_4kp60 as 99% of 4K HEVC media in circulation is 4K@30 or [email protected] and thus 4K60 which forces the SoC to run permanently in a more resource-demanding mode is not normally required.
I run an RPi4 as the family daily-driver without any real-world issues. It's miles more reliable than other devices I have.