For words that only have three letters, there is no 4th letter. As an example with the word “include”, there aren't any a lot more letters afterwards and therefore needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however you would have applied “addi” for https://myeasybookmarks.com/story4293099/not-known-factual-statements-about-bip39-explained