For words that have only three letters, there is absolutely no 4th letter. For instance with the word “include”, there are no more letters afterwards and so needs to be the word "add". The word “addict” is within the BIP39 wordlist but you would've employed “addi” for “addict”. One more https://ztndz.com/story21924984/helping-the-others-realize-the-advantages-of-bip39-mnemonic-phrase-tool