For words that only have 3 letters, there is not any 4th letter. By way of example with the word “include”, there isn't any extra letters afterwards and so has to be the word "increase". The word “addict” is during the BIP39 wordlist however you would've made use of “addi” https://linkedbookmarker.com/story4279952/a-review-of-bip39-phrase-list