For words that only have three letters, there is absolutely no 4th letter. For instance with the word “add”, there won't be any a lot more letters Later on and thus needs to be the word "add". The word “addict” is from the BIP39 wordlist but you might have made http://edgargvkh682.theburnward.com/from-mnemonics-to-security-understanding-bip39-passphrases-and-their-importance-in-copyright