input: Write a python function to clean a string by removing non-alphanumeric characters, converting to lowercase, and filtering out words with 3 or fewer characters.
  assert clean_string("This is, MY string!?") == "this string"
  assert clean_string("Hello, World! This is a Test.") == "hello world this test"
  assert clean_string("A quick brown fox jumps over the lazy dog.") == "quick brown jumps over lazy"
output: def clean_string(s):
      import re
      words = re.findall(r'\b\w{4,}\b', s.lower())
      return ' '.join(words)