For words that only have 3 letters, there is absolutely no 4th letter. As an example with the word “increase”, there are no additional letters afterwards and thus has to be the word "include". The word “addict” is inside the BIP39 wordlist however, you might have applied “addi” for “addict”. https://bookmarkchamp.com/story19547691/5-essential-elements-for-bip39-explained