input: Write a function to filter a list such that each element is at least 4 units larger than the previous element.
  assert solution([1, 2, 4, 5, 8, 14, 15]) == [1, 4, 8, 14]
  assert solution([3, 7, 10, 14, 18, 20]) == [3, 7, 14]
  assert solution([1, 2, 3, 7, 8, 12, 16, 19]) == [1, 7, 12, 19]
output: def solution(lst):
      if not lst:
          return []
      
      result = [lst[0]]
      for num in lst[1:]:
          if num >= result[-1] + 4:
              result.append(num)
      return result