For words that have only three letters, there isn't a 4th letter. One example is with the word “insert”, there won't be any a lot more letters afterwards and thus must be the word "add". The word “addict” is inside the BIP39 wordlist however you would've utilized “addi” for “addict”. https://bookmarkity.com/story19104977/a-review-of-bip39-phrase-list