For words that only have 3 letters, there's no 4th letter. As an example with the word “include”, there aren't any much more letters Later on and therefore needs to be the word "include". The word “addict” is within the BIP39 wordlist however, you would have utilized “addi” for “addict”. https://bookmarkworm.com/story18990508/facts-about-bip39-seed-phrase-github-revealed