For words that have only 3 letters, there is no 4th letter. As an example with the word “insert”, there aren't any additional letters afterwards and therefore must be the word "increase". The word “addict” is from the BIP39 wordlist however you would have applied “addi” for “addict”. A seed https://esocialmall.com/story4197216/a-secret-weapon-for-bip39-words-to-numbers