The Raspberry Pi 4 is natively supported out-of-the-box by the Thistle Update Client, with no further configuration required.
As for any bootloader setup supported by the Thistle Update Client, it is required to have 2 partitions available, that will be used as root filesystem alternatively. SD card re-partitioning might be necessary, see the Raspberry Pi get-started guide.
Once set in the configuration, the two partitions will be automatically detected and the Update Client will instruct the bootloader automatically. See below the extract of an example configuration supporting Raspberry Pi - the bootloader is set to RaspberryPi and the two partitions a and b are specified. 
$ cat config.json
{
    ...
    "bootloader": "RaspberryPi",
    "part_a": "/dev/mmcblk0p2",
    "part_b": "/dev/mmcblk0p3",
}