For words that have only three letters, there's no 4th letter. By way of example with the word “include”, there won't be any far more letters afterwards and therefore needs to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would've used “addi” for “addict”. https://yxzbookmarks.com/story18813679/not-known-facts-about-bip39-english-words