input: Write a function to format a given piece of text with different options for font-style, size, and color. The function should have parameters for text, font-style, size, and color, and return the formatted text as an HTML span.
  assert format_text('Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer ut lectus vel nisi consequat bibendum ac a erat. Aliquam non velit sit amet arcu placerat vulputate a ut nibh.', 'Times New Roman', '14px', '#000000')=='<span style=\'font-family: Times New Roman\'>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Integer ut lectus vel nisi consequat bibendum ac a erat. Aliquam non velit sit amet arcu placerat vulputate a ut nibh.</span>'
  assert format_text('Hello World', 'Arial', '20px', '#FF0000')=='<span style=\'font-family: Arial\'>Hello World</span>'
  assert format_text('This is a test', 'Courier New', '16px', '#00FF00')=='<span style=\'font-family: Courier New\'>This is a test</span>'
  assert format_text('Another example', 'Verdana', '12px', '#0000FF')=='<span style=\'font-family: Verdana\'>Another example</span>'
  assert format_text('Unit Test', 'Times New Roman', '10px', '#808080')=='<span style=\'font-family: Times New Roman\'>Unit Test</span>'
output: def format_text(text, font_style, size, color):
      return f'<span style=\'font-family: {font_style}; font-size: {size}; color: {color}\'>{text}</span>'