To do it the lazy but more risk-prone way, you need to SSH in and run three commands:
touch /storage/.update/.nocompat
wget http://releases.libreelec.tv/LibreELEC-RPi4.arm-9.2.0.tar
reboot
This will disable the compat check (else we detect RPi4 is the wrong image for your RPi2(3) device and abort update) and then we download the RPi4 image and reboot. On reboot it will update to the RPi4 image. On reboot after updating it should fail to boot. You can now power off the RPi3 and move the SD card to the RPi4, which should boot. Remove any overclock/tuning etc. from config.txt before booting the RPi4 - it's not necessary.
If you want to play safe, make a backup first, then you can always clean install the RPi4 and restore the previous configuration.