For words that have only three letters, there's no 4th letter. As an example with the word “add”, there isn't any a lot more letters Later on and thus must be the word "add". The word “addict” is during the BIP39 wordlist however you would have used “addi” for “addict”. https://thesocialdelight.com/story4995901/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting