For words that only have three letters, there isn't any 4th letter. Such as with the word “incorporate”, there isn't any additional letters afterwards and so have to be the word "insert". The word “addict” is within the BIP39 wordlist however you would've employed “addi” for “addict”. BIP39 and 44, https://bookmarksusa.com/story18883337/the-greatest-guide-to-seed-phrase-bip39