For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “include”, there isn't any additional letters afterwards and so must be the word "incorporate". The word “addict” is within the BIP39 wordlist however, you would have utilised “addi” for “addict”. https://doctorbookmark.com/story18999642/bip39-seed-phrase-fundamentals-explained