For words that have only three letters, there is no 4th letter. One example is with the word “include”, there are no more letters afterwards and so needs to be the word "add". The word “addict” is within the BIP39 wordlist however, you might have used “addi” for “addict”. Shop https://tripsbookmarks.com/story19071005/new-step-by-step-map-for-bip39-english-words