For words that only have 3 letters, there isn't any 4th letter. As an example with the word “increase”, there aren't any additional letters afterwards and thus should be the word "add". The word “addict” is during the BIP39 wordlist however you would have made use of “addi” for “addict”. https://bookmarkedblog.com/story19773432/a-review-of-bip39