For words that have only 3 letters, there is not any 4th letter. As an example with the word “increase”, there are no extra letters afterwards and so has to be the word "include". The word “addict” is from the BIP39 wordlist however, you would have applied “addi” for “addict”. https://bookmarkinglive.com/story19638109/new-step-by-step-map-for-bip39-english-words