FDE without TPM requires the passphrase to have as many bits of security as you want the encryption to have. With a TPM you can have something as short as a PIN, since the TPM can limit the rate at which password attempts are made. I assume most users are satisfied with the weak security implied by a short passphrase, but a dedicated attacker could likely crack it.

FDE uses a key derivation function to limit the rate at which offline password attempts can be made. Not as limited as a TPM can be but not unlimited either and you can adjust it depending on how long you want to wait every time you unlock.

So for example "4" would work ?