For words that only have three letters, there isn't any 4th letter. One example is with the word “increase”, there isn't any more letters Later on and so should be the word "increase". The word “addict” is inside the BIP39 wordlist however, you might have used “addi” for “addict”. The https://cruxbookmarks.com/story18951637/a-review-of-bip39-passphrase