For words that have only 3 letters, there's no 4th letter. By way of example with the word “add”, there aren't any far more letters afterwards and thus need to be the word "insert". The word “addict” is during the BIP39 wordlist however, you would have applied “addi” for “addict”. https://nanobookmarking.com/story18872991/the-invalid-bip39-phrase-diaries