For words that have only three letters, there's no 4th letter. As an example with the word “increase”, there are no more letters afterwards and thus must be the word "insert". The word “addict” is in the BIP39 wordlist however , you would have applied “addi” for “addict”. Tips on https://socialwebleads.com/story4477388/the-basic-principles-of-bip39-passphrase-generator