For words that only have three letters, there's no 4th letter. As an example with the word “incorporate”, there are no extra letters Later on and thus needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would've employed “addi” for “addict”. Standard https://bookmarkshut.com/story19693883/5-simple-techniques-for-bip39-seed-phrase