For words that only have three letters, there is not any 4th letter. By way of example with the word “include”, there isn't any additional letters afterwards and so must be the word "increase". The word “addict” is from the BIP39 wordlist but you would've applied “addi” for “addict”. Using https://wakelet.com/wake/XIloocQnhZGti1r1O9tJb