For words that have only three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there aren't any extra letters afterwards and so should be the word "insert". The word “addict” is during the BIP39 wordlist however you would have utilized “addi” for “addict”. https://bookmarkblast.com/story18858621/the-ultimate-guide-to-bip39-seed-phrase-mnemonics-generator