For words that have only three letters, there's no 4th letter. For example with the word “add”, there won't be any much more letters Later on and so must be the word "add". The word “addict” is while in the BIP39 wordlist however you would have applied “addi” for “addict”. https://opensocialfactory.com/story19301462/a-secret-weapon-for-bip39-words-to-numbers