For words that only have three letters, there is not any 4th letter. For instance with the word “add”, there aren't any additional letters Later on and therefore need to be the word "increase". The word “addict” is in the BIP39 wordlist however, you would have used “addi” for “addict”. https://bookmarkcork.com/story19407712/the-2-minute-rule-for-bip39-all-words