For words that only have three letters, there is absolutely no 4th letter. For instance with the word “insert”, there aren't any additional letters afterwards and therefore has to be the word "incorporate". The word “addict” is during the BIP39 wordlist however you would have used “addi” for “addict”. Should https://bookmarkmiracle.com/story21294965/the-definitive-guide-to-bip39-seed-phrase