For words that have only three letters, there's no 4th letter. For instance with the word “incorporate”, there won't be any extra letters afterwards and so needs to be the word "insert". The word “addict” is inside the BIP39 wordlist however , you would have employed “addi” for “addict”. Mnemonic https://sociallweb.com/story4266473/bip39-seed-phrase-fundamentals-explained