For words that only have 3 letters, there is absolutely no 4th letter. One example is with the word “incorporate”, there are no additional letters Later on and so have to be the word "insert". The word “addict” is inside the BIP39 wordlist however you would've applied “addi” for “addict”. https://ledbookmark.com/story4486830/the-ultimate-guide-to-bip39-seed-phrase-github