For words that only have three letters, there's no 4th letter. As an example with the word “add”, there won't be any a lot more letters afterwards and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however you would have applied “addi” for https://bookmarktune.com/story18758131/considerations-to-know-about-bip39-seed-phrase-generator