For words that have only three letters, there is absolutely no 4th letter. One example is with the word “insert”, there isn't any more letters Later on and so should be the word "add". The word “addict” is within the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://pr8bookmarks.com/story18936897/the-ultimate-guide-to-bip39-seed-phrase-github