For words that only have 3 letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any additional letters afterwards and so need to be the word "add". The word “addict” is while in the BIP39 wordlist however you would have employed “addi” for https://topsocialplan.com/story4551340/the-definitive-guide-to-bip39-seed-phrase