For words that only have 3 letters, there is absolutely no 4th letter. For example with the word “add”, there isn't any a lot more letters Later on and so need to be the word "insert". The word “addict” is while in the BIP39 wordlist however you would've used “addi” https://wiishlist.com/story19644936/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting