For words that only have three letters, there isn't any 4th letter. For instance with the word “insert”, there won't be any more letters afterwards and therefore has to be the word "insert". The word “addict” is within the BIP39 wordlist however , you would have utilised “addi” for “addict”. https://minibookmarks.com/story19453154/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator