For words that have only 3 letters, there's no 4th letter. One example is with the word “include”, there are no a lot more letters afterwards and therefore needs to be the word "add". The word “addict” is while in the BIP39 wordlist however, you would've used “addi” for “addict”. https://thebookmarklist.com/story18778499/considerations-to-know-about-bip39-seed-phrase-generator