For words that only have three letters, there is absolutely no 4th letter. As an example with the word “include”, there aren't any much more letters afterwards and therefore needs to be the word "insert". The word “addict” is in the BIP39 wordlist but you might have applied “addi” for https://bookmarkfly.com/story18850829/the-basic-principles-of-bip39-passphrase-generator