For words that only have 3 letters, there's no 4th letter. By way of example with the word “include”, there isn't any extra letters afterwards and so need to be the word "include". The word “addict” is in the BIP39 wordlist however , you would have made use of “addi” https://sociallytraffic.com/story3765180/new-step-by-step-map-for-bip39-english-words