For words that only have 3 letters, there isn't any 4th letter. For example with the word “incorporate”, there won't be any additional letters Later on and therefore have to be the word "add". The word “addict” is inside the BIP39 wordlist however, you would have used “addi” for “addict”. https://ticketsbookmarks.com/story18867140/a-review-of-bip39-passphrase