For words that have only 3 letters, there is absolutely no 4th letter. One example is with the word “insert”, there aren't any much more letters afterwards and so need to be the word "include". The word “addict” is in the BIP39 wordlist however, you would've employed “addi” for “addict”. https://listfav.com/story20323065/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting