For words that have only 3 letters, there's no 4th letter. Such as with the word “insert”, there won't be any much more letters Later on and thus needs to be the word "insert". The word “addict” is during the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://www.openlearning.com/u/isabelmorgan-sqa4a6/about/