For words that have only 3 letters, there is not any 4th letter. For example with the word “incorporate”, there are no additional letters afterwards and so has to be the word "include". The word “addict” is from the BIP39 wordlist however you would have made use of “addi” for https://thesocialvibes.com/story4643482/facts-about-bip39-seed-phrase-word-list-revealed