For words that only have three letters, there is absolutely no 4th letter. By way of example with the word “insert”, there are no additional letters Later on and therefore has to be the word "insert". The word “addict” is within the BIP39 wordlist however, you might have utilized “addi” https://wiishlist.com/story19437461/detailed-notes-on-copyright-wallet-seed-phrase-out-of-bip39