For words that have only 3 letters, there's no 4th letter. For example with the word “add”, there isn't any extra letters afterwards and thus must be the word "insert". The word “addict” is during the BIP39 wordlist however, you might have utilized “addi” for “addict”. BIP39 and 44, Conversely, https://bookmarksden.com/story19106533/the-2-minute-rule-for-bip39-all-words