She knew:

• It had 4 or 5 digits.
• It used 4 or 5 different digits.
• The first digit is 4 or 5.
• The second digit is 4 or 5 and none of the other digits is smaller.
• The number is 4 or 5 times a prime number.
• If you reverse the order of the digits, the resulting number has 4 or 5 prime factors, all different.
• In 4 or 5 of these facts, the "4" is the correct number.

What is Susan's PIN code?

"Are we done now, Charlie?"

When it's narrowed down to one answer, clearing the ambiguity.