For words that only have 3 letters, there isn't any 4th letter. One example is with the word “add”, there aren't any additional letters Later on and thus must be the word "incorporate". The word “addict” is within the BIP39 wordlist however , you would have utilised “addi” for “addict”. https://www.storeboard.com/blogs/beauty-and-fashion/opening-the-globe-of-bip39-a-comprehensive-guide-to-mnemonic-expressions-and-word-checklists/6026188