For words that only have three letters, there isn't any 4th letter. Such as with the word “add”, there are no far more letters Later on and so has to be the word "insert". The word “addict” is from the BIP39 wordlist but you might have employed “addi” for “addict”. https://myfirstbookmark.com/story19172515/the-ultimate-guide-to-bip39-seed-phrase-github