Libreelec 10.3 on RP4 no longer boots, how can I upgrade or solve this?

Your Windows PC can only read the VFAT partition of the microSD. However, you want to copy the .kodi folder to USB, which is at the EXT4 partition.

To read and copy EXT4 content, create a bootable Ubuntu stick:

https://ubuntu.com/tutorials/try-ubuntu-before-you-install#1-getting-started

LE is largely packaged into two large compressed files (KERNEL and SYSTEM). If anything corrupts them, they cannot decompress and LE won't boot, whereas with a more conventionally packaged OS with thousands of files scattered over the disk; if you corrupt a couple of files it will usually manage to boot and you might not notice the things that failed due to the corruption.

Cards with corrupted filesystems can be fixed with 'fsck' from the USB stick install or another distro. If you can fix the filesystem you can boot from the card again, and/or copy data off the card to make a backup. Cards can also fail with dead memory breaking the filesystems. This cannot be fixed with fsck, so if fsck fails to repair the card or hangs .. you lost data (and learned why taking backups is a good idea) and you need to start over with a new card (or remain on the USB stick).

Boot the RPi4 from USB, then connect the SD card. Then SSH into the RPi4 from Windows, unmount the filesystems on the SD card so they are not mounted (in use) and filesystem check (fsck) them:

umount /dev/mmcblk0p1
umount /dev/mmcblk0p2
fsck -y /dev/mmbclk0p1
fsck -y /dev/mmcblk0p2

See how that goes.

My 1st learned computer experience:

"The mean time to the next computer storage failure is directly proportional to the importance of the data."