For words that have only 3 letters, there's no 4th letter. As an example with the word “add”, there won't be any more letters Later on and so need to be the word "add". The word “addict” is while in the BIP39 wordlist however you would've used “addi” for “addict”. https://express-page.com/story4326560/considerations-to-know-about-bip39-seed-phrase-generator