For words that only have 3 letters, there is no 4th letter. For example with the word “insert”, there won't be any a lot more letters afterwards and so need to be the word "incorporate". The word “addict” is from the BIP39 wordlist however , you would have used “addi” https://bookmarkspy.com/story20433267/the-definitive-guide-to-bip39-seed-phrase