For words that only have 3 letters, there is no 4th letter. One example is with the word “insert”, there are no additional letters Later on and therefore has to be the word "insert". The word “addict” is in the BIP39 wordlist however , you would've utilised “addi” for “addict”. https://sitesrow.com/story8605539/the-2-minute-rule-for-bip39-all-words