For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there won't be any extra letters Later on and thus needs to be the word "include". The word “addict” is inside the BIP39 wordlist however you might have applied https://loanbookmark.com/story18882403/examine-this-report-on-bip39-mnemonic-phrase-tool