For words that have only 3 letters, there is no 4th letter. For example with the word “insert”, there won't be any additional letters Later on and therefore needs to be the word "insert". The word “addict” is in the BIP39 wordlist however you would have utilized “addi” for “addict”. https://ilovebookmarking.com/story18832078/phrases-with-bit-an-overview