For words that only have three letters, there isn't any 4th letter. For instance with the word “insert”, there are no additional letters afterwards and so have to be the word "incorporate". The word “addict” is from the BIP39 wordlist however you might have employed “addi” for “addict”. BIP39 and https://trackbookmark.com/story20265766/an-unbiased-view-of-bip39-seed-phrase-github