For words that have only 3 letters, there is not any 4th letter. By way of example with the word “include”, there are no extra letters afterwards and so have to be the word "incorporate". The word “addict” is inside the BIP39 wordlist however , you would have used “addi” https://getidealist.com/story20655765/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator