For words that only have 3 letters, there's no 4th letter. For instance with the word “insert”, there won't be any much more letters Later on and so need to be the word "include". The word “addict” is inside the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://throbsocial.com/story20671864/5-essential-elements-for-bip39-explained