For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there aren't any a lot more letters afterwards and so needs to be the word "increase". The word “addict” is from the BIP39 wordlist however, you might have applied “addi” for “addict”. https://echobookmarks.com/story19007818/top-guidelines-of-bip39-word-list-pdf