For words that only have 3 letters, there is absolutely no 4th letter. By way of example with the word “insert”, there aren't any additional letters afterwards and therefore need to be the word "incorporate". The word “addict” is from the BIP39 wordlist however you might have utilised “addi” for https://bookmarkingalpha.com/story19614291/getting-my-bip39-words-to-numbers-to-work