For words that have only three letters, there isn't a 4th letter. By way of example with the word “insert”, there are no far more letters afterwards and thus must be the word "increase". The word “addict” is during the BIP39 wordlist however, you would've applied “addi” for “addict”. By https://lingeriebookmark.com/story8867289/top-guidelines-of-bip39-word-list-pdf