For words that have only 3 letters, there is no 4th letter. For instance with the word “include”, there isn't any extra letters afterwards and so needs to be the word "add". The word “addict” is in the BIP39 wordlist however, you would've made use of “addi” for “addict”. bip39-phrase.com https://socialexpresions.com/story4998907/a-review-of-bip39-phrase-list