For words that only have 3 letters, there isn't any 4th letter. One example is with the word “add”, there isn't any extra letters afterwards and thus needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would've utilized “addi” for “addict”. You https://geniusbookmarks.com/story18978264/not-known-facts-about-bip39-english-words