For words that have only three letters, there isn't any 4th letter. For instance with the word “add”, there aren't any far more letters afterwards and therefore has to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://checkbookmarks.com/story5187529/5-essential-elements-for-bip39-explained