For words that have only three letters, there's no 4th letter. Such as with the word “include”, there aren't any far more letters afterwards and therefore needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however , you would have used “addi” for “addict”. The https://ledbookmark.com/story5287306/5-essential-elements-for-bip39-explained