If you've tried to install Lakka from a NOOBS micro-SD card or via the Lakka website directly, please note that (at the time of writing) the current stable release of Lakka does not support the 8GB Raspberry Pi 4.
However, 'nightly' builds are available which have the up to date kernel that the 8GB Raspberry Pi requires. Please remember that nightly builds are not stable releases and may (or may not) have bugs and other issues.