For words that only have three letters, there isn't a 4th letter. By way of example with the word “incorporate”, there aren't any much more letters Later on and thus should be the word "incorporate". The word “addict” is in the BIP39 wordlist however , you might have used “addi” https://yoursocialpeople.com/story4186638/considerations-to-know-about-bip39-word-list-pdf