For words that have only 3 letters, there isn't any 4th letter. As an example with the word “add”, there isn't any a lot more letters afterwards and therefore must be the word "add". The word “addict” is within the BIP39 wordlist however, you would have used “addi” for “addict”. https://todaybookmarks.com/story18920980/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator