For words that only have three letters, there's no 4th letter. For example with the word “insert”, there aren't any extra letters afterwards and therefore should be the word "insert". The word “addict” is from the BIP39 wordlist however you would have utilised “addi” for “addict”. BIP39 and forty four, https://bookmarknap.com/story9078410/not-known-factual-statements-about-bip39-explained