For words that have only 3 letters, there is not any 4th letter. By way of example with the word “add”, there aren't any extra letters Later on and so has to be the word "add". The word “addict” is within the BIP39 wordlist but you might have utilized “addi” https://wiishlist.com/story19406563/bip39-phrase-generator-can-be-fun-for-anyone