For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “insert”, there aren't any additional letters Later on and so should be the word "include". The word “addict” is in the BIP39 wordlist however you would have utilized “addi” for “addict”. https://bookmarkinglog.com/story19475541/a-review-of-bip39-phrase-list