For words that have only three letters, there isn't a 4th letter. One example is with the word “add”, there are no more letters afterwards and thus has to be the word "increase". The word “addict” is inside the BIP39 wordlist however, you would've utilized “addi” for “addict”. Mnemonic words https://bookmarkcork.com/story19629221/top-guidelines-of-bip39-word-list-pdf