Download release from https://downloads.openwrt.org/releases/21.02.0/targets/bcm27xx/bcm2711/openwrt-21.02.0-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
gunzip openwrt-21.02.0-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
dd if=openwrt-21.02.0-bcm27xx-bcm2711-rpi-4-ext4-factory.img of=/dev/SDCARD status=progress
mount rootfs to your pc