For words that only have 3 letters, there is not any 4th letter. By way of example with the word “insert”, there are no much more letters Later on and therefore needs to be the word "increase". The word “addict” is inside the BIP39 wordlist however you would have employed https://top10bookmark.com/story19477181/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition