For words that only have 3 letters, there isn't any 4th letter. For example with the word “insert”, there aren't any extra letters afterwards and so have to be the word "add". The word “addict” is from the BIP39 wordlist however you would've employed “addi” for “addict”. This extra complexity https://toplistar.com/story20638390/top-latest-five-bip39-words-urban-news