For words that have only 3 letters, there isn't any 4th letter. One example is with the word “insert”, there are no much more letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however you would've utilized “addi” for “addict”. In https://bookmarkswing.com/story20407453/the-invalid-bip39-phrase-diaries