For words that only have three letters, there is absolutely no 4th letter. For instance with the word “insert”, there won't be any additional letters afterwards and thus have to be the word "insert". The word “addict” is within the BIP39 wordlist however you would have used “addi” for “addict”. https://thesocialroi.com/story8583998/a-review-of-bip39-phrase-list