For words that only have 3 letters, there is not any 4th letter. For example with the word “include”, there aren't any much more letters Later on and thus must be the word "increase". The word “addict” is during the BIP39 wordlist however, you would have utilized “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