For words that have only three letters, there isn't any 4th letter. For example with the word “insert”, there aren't any more letters afterwards and so have to be the word "add". The word “addict” is inside the BIP39 wordlist but you might have employed “addi” for “addict”. Standard recovery https://linkingbookmark.com/story18702063/the-invalid-bip39-phrase-diaries