For words that have only 3 letters, there is absolutely no 4th letter. One example is with the word “increase”, there aren't any a lot more letters Later on and thus need to be the word "include". The word “addict” is in the BIP39 wordlist however, you might have applied https://mirrorbookmarks.com/story19535523/not-known-facts-about-bip39-english-words