For words that have only 3 letters, there is not any 4th letter. By way of example with the word “add”, there are no additional letters afterwards and therefore must be the word "incorporate". The word “addict” is in the BIP39 wordlist however you might have used “addi” for “addict”. https://mysocialport.com/story5067022/considerations-to-know-about-bip39-word-list-pdf