For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there isn't any more letters Later on and thus need to be the word "increase". The word “addict” is while in the BIP39 wordlist however you would have used https://letusbookmark.com/story20435278/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting