For words that have only 3 letters, there isn't any 4th letter. For example with the word “add”, there aren't any much more letters Later on and so has to be the word "include". The word “addict” is during the BIP39 wordlist however , you would have employed “addi” for https://cheapbookmarking.com/story18945879/not-known-factual-statements-about-bip39-explained