For words that only have three letters, there is no 4th letter. One example is with the word “add”, there won't be any more letters Later on and therefore should be the word "include". The word “addict” is in the BIP39 wordlist however , you would've utilized “addi” for “addict”. https://socialwebconsult.com/story4895608/5-essential-elements-for-bip39-explained