For words that have only 3 letters, there's no 4th letter. One example is with the word “add”, there won't be any far more letters afterwards and thus should be the word "add". The word “addict” is in the BIP39 wordlist however , you would have made use of “addi” https://royalbookmarking.com/story19021452/a-review-of-bip39-passphrase