For words that only have 3 letters, there's no 4th letter. By way of example with the word “add”, there isn't any much more letters afterwards and so have to be the word "add". The word “addict” is while in the BIP39 wordlist however, you would've utilized “addi” for “addict”. https://hubwebsites.com/story20159550/considerations-to-know-about-bip39-word-list-pdf