For words that only have three letters, there is not any 4th letter. By way of example with the word “include”, there are no extra letters afterwards and so have to be the word "add". The word “addict” is while in the BIP39 wordlist however you would've used “addi” for https://fellowfavorite.com/story19989215/invalid-bip39-phrase-things-to-know-before-you-buy