For words that have only 3 letters, there isn't any 4th letter. Such as with the word “incorporate”, there are no additional letters afterwards and therefore should be the word "insert". The word “addict” is in the BIP39 wordlist however, you would have utilised “addi” for “addict”. An easy backup https://thejillist.com/story8909809/top-latest-five-bip39-words-urban-news