For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any much more letters afterwards and so have to be the word "increase". The word “addict” is from the BIP39 wordlist however you would have used “addi” for https://socialmediainuk.com/story20634229/the-basic-principles-of-bip39-passphrase-generator