input: Write a function to find all 4-letter words in a given string.
  assert find_words('The quick brown fox jumps over the lazy dog') == ['quick', 'brown', 'over', 'lazy']
  assert find_words('This is a test string with some four-letter words') == ['This', 'test', 'with', 'some']
  assert find_words('No four letter words here') == []
output: def find_words(s):
      return [word for word in s.split() if len(word) == 4]