For words that only have 3 letters, there isn't a 4th letter. One example is with the word “increase”, there won't be any extra letters afterwards and therefore need to be the word "add". The word “addict” is inside the BIP39 wordlist however, you might have utilized “addi” for “addict”. https://thebookmarkplaza.com/story19532601/examine-this-report-on-bip39-passphrase