For words that have only 3 letters, there's no 4th letter. As an example with the word “include”, there isn't any far more letters afterwards and thus needs to be the word "increase". The word “addict” is from the BIP39 wordlist however, you might have applied “addi” for “addict”. Standard https://bookmarkbells.com/story18969776/phrases-with-bit-an-overview