For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “include”, there isn't any additional letters afterwards and so needs to be the word "add". The word “addict” is within the BIP39 wordlist however you might have used “addi” for “addict”. Many https://wildbookmarks.com/story19013603/the-2-minute-rule-for-bip39-all-words