For words that have only 3 letters, there is absolutely no 4th letter. Such as with the word “include”, there are no more letters Later on and so have to be the word "insert". The word “addict” is during the BIP39 wordlist however, you would have made use of “addi” https://bookmarkstime.com/story19325069/the-fact-about-valid-bip39-seed-phrase-that-no-one-is-suggesting