For words that only have 3 letters, there is absolutely no 4th letter. For instance with the word “include”, there aren't any far more letters Later on and thus needs to be the word "insert". The word “addict” is inside the BIP39 wordlist however you would've employed “addi” for “addict”. http://andersonsaeu988.iamarrows.com/from-mnemonics-to-safety-and-security-recognizing-bip39-passphrases-and-their-significance-in-copyright