For words that have only three letters, there is no 4th letter. By way of example with the word “include”, there are no additional letters afterwards and so have to be the word "include". The word “addict” is during the BIP39 wordlist however you would've made use of “addi” for https://maroonbookmarks.com/story18814812/new-step-by-step-map-for-bip39-english-words