For words that have only 3 letters, there is absolutely no 4th letter. For instance with the word “add”, there are no far more letters afterwards and thus should be the word "add". The word “addict” is during the BIP39 wordlist however , you would've utilized “addi” for “addict”. Mnemonic https://exactlybookmarks.com/story18988009/examine-this-report-on-bip39-passphrase