For words that have only 3 letters, there is absolutely no 4th letter. By way of example with the word “add”, there are no extra letters afterwards and thus need to be the word "include". The word “addict” is during the BIP39 wordlist however, you would have utilised “addi” for https://socialtechnet.com/story4257359/5-essential-elements-for-bip39-explained