For words that only have three letters, there's no 4th letter. For instance with the word “increase”, there aren't any extra letters afterwards and thus have to be the word "add". The word “addict” is from the BIP39 wordlist but you would have used “addi” for “addict”. Shop your phrase https://bookmarkyourpage.com/story4254837/the-basic-principles-of-bip39-passphrase-generator