For words that have only 3 letters, there's no 4th letter. As an example with the word “increase”, there are no more letters afterwards and thus has to be the word "increase". The word “addict” is from the BIP39 wordlist however, you would have applied “addi” for “addict”. Standard recovery https://bookmarkbooth.com/story19166028/a-secret-weapon-for-bip39-words-to-numbers