For words that only have 3 letters, there is no 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 inside the BIP39 wordlist however , you would have utilised “addi” for “addict”. In https://linkedbookmarker.com/story4332939/the-2-minute-rule-for-bip39-all-words