For words that have only three letters, there is no 4th letter. As an example with the word “incorporate”, there isn't any a lot more letters Later on and thus has to be the word "add". The word “addict” is within the BIP39 wordlist however, you might have applied “addi” http://edgargvkh682.theburnward.com/from-mnemonics-to-security-understanding-bip39-passphrases-and-their-importance-in-copyright