For words that only have three letters, there's no 4th letter. As an example with the word “add”, there isn't any extra letters afterwards and so needs to be the word "insert". The word “addict” is during the BIP39 wordlist however , you would've employed “addi” for “addict”. You might https://bookmarkloves.com/story20825352/not-known-factual-statements-about-bip39-explained