For words that only have 3 letters, there's no 4th letter. As an example with the word “increase”, there are no a lot more letters afterwards and so needs to be the word "insert". The word “addict” is in the BIP39 wordlist but you would've applied “addi” for “addict”. Obtaining https://postheaven.net/tiablexcen/mastering-your-copyright-protection-the-essential-overview-to-bip39-passphrases