For words that have only 3 letters, there isn't any 4th letter. As an example with the word “include”, there are no additional letters afterwards and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however you would have used “addi” for “addict”. Multiple https://social-galaxy.com/story4256800/new-step-by-step-map-for-bip39-english-words