For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any extra letters afterwards and so need to be the word "add". The word “addict” is during the BIP39 wordlist however you would've made use of “addi” for “addict”. The https://advicebookmarks.com/story26518071/bip39-seed-phrase-fundamentals-explained