For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “insert”, there won't be any more letters afterwards and therefore should be the word "include". The word “addict” is during the BIP39 wordlist however you would have utilized “addi” for “addict”. https://altbookmark.com/story20445130/the-2-minute-rule-for-bip39-all-words