For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there won't be any additional letters afterwards and therefore need to be the word "insert". The word “addict” is while in the BIP39 wordlist but you might have used http://rylanmecp341.wpsuo.com/from-words-to-wide-range-exactly-how-to-efficiently-make-use-of-bip39-passphrases-for-your-copyright-pocketbooks