For words that have only three letters, there isn't any 4th letter. As an example with the word “insert”, there aren't any far more letters afterwards and thus have to be the word "add". The word “addict” is during the BIP39 wordlist however you would have applied “addi” for “addict”. https://moodjhomedia.com/story2657095/little-known-facts-about-bip39-words