For words that have only three letters, there isn't a 4th letter. As an example with the word “increase”, there isn't any more letters Later on and therefore need to be the word "add". The word “addict” is during the BIP39 wordlist however , you would've made use of “addi” https://postheaven.net/tiablexcen/mastering-your-copyright-protection-the-essential-overview-to-bip39-passphrases