For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there are no additional letters afterwards and thus has to be the word "include". The word “addict” is while in the BIP39 wordlist but you might have employed “addi” for “addict”. This https://totalbookmarking.com/story18988371/the-invalid-bip39-phrase-diaries