For words that only have 3 letters, there isn't any 4th letter. For example with the word “add”, there isn't any far more letters afterwards and so has to be the word "increase". The word “addict” is within the BIP39 wordlist however , you would have used “addi” for “addict”. https://baidubookmark.com/story18774958/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting