For words that only have three letters, there's no 4th letter. As an example with the word “incorporate”, there aren't any much more letters afterwards and so has to be the word "include". The word “addict” is while in the BIP39 wordlist however, you might have utilized “addi” for “addict”. https://bookmarkbirth.com/story19741377/not-known-facts-about-bip39-english-words