For words that only have 3 letters, there isn't any 4th letter. For instance with the word “increase”, there won't be any extra letters afterwards and so needs to be the word "increase". The word “addict” is during the BIP39 wordlist however, you would have used “addi” for “addict”. Discovering https://total-bookmark.com/story18701199/phrases-with-bit-an-overview