For words that have only 3 letters, there isn't any 4th letter. For example with the word “insert”, there are no extra letters afterwards and thus must be the word "insert". The word “addict” is within the BIP39 wordlist however , you would have applied “addi” for “addict”. On this https://thebookmarkage.com/story18927262/top-latest-five-bip39-seed-phrase-mnemonics-generator-urban-news