For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there aren't any much more letters afterwards and so needs to be the word "include". The word “addict” is inside the BIP39 wordlist but you might have made use of “addi” https://total-bookmark.com/story19344914/not-known-facts-about-bip39-english-words