For words that have only 3 letters, there isn't any 4th letter. By way of example with the word “increase”, there aren't any additional letters afterwards and therefore need to be the word "increase". The word “addict” is in the BIP39 wordlist however, you would have employed “addi” for “addict”. https://businessbookmark.com/story4225169/considerations-to-know-about-bip39-word-list-pdf