For words that only have three letters, there is absolutely no 4th letter. As an example with the word “add”, there are no far more letters Later on and therefore needs to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however you would've applied “addi” for https://ztndz.com/story22055568/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting