For words that have only 3 letters, there is absolutely no 4th letter. By way of example with the word “increase”, there isn't any much more letters afterwards and so needs to be the word "insert". The word “addict” is while in the BIP39 wordlist however , you might have https://ezmarkbookmarks.com/story19135971/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting