For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “add”, there isn't any much more letters afterwards and so need to be the word "add". The word “addict” is during the BIP39 wordlist however you would have employed “addi” https://whitebookmarks.com/story19088241/bip39-phrase-generator-can-be-fun-for-anyone