For words that have only three letters, there is absolutely no 4th letter. One example is with the word “insert”, there isn't any far more letters afterwards and so has to be the word "increase". The word “addict” is during the BIP39 wordlist but you would've utilized “addi” for “addict”. https://bookmarkgenious.com/story18975465/the-invalid-bip39-phrase-diaries