For words that have only three letters, there is no 4th letter. As an example with the word “include”, there aren't any additional letters afterwards and thus need to be the word "insert". The word “addict” is in the BIP39 wordlist however , you would have used “addi” for “addict”. https://bookmarksystem.com/story18860610/the-greatest-guide-to-seed-phrase-bip39