Given a credit card number as a string, mask all characters except the last 4 by replacing them with the '#' character. Return null for null input and an empty string for empty input. If the string has 4 or fewer characters, return it unchanged.

Examples:

Input: cardNumber = '4111111111111111' Output: '############1111' Explanation: All characters except the last 4 are replaced with '#' Input: cardNumber = '12345' Output: '#2345' Explanation: Only the first character is masked, last 4 are preserved Input: cardNumber = '1234' Output: '1234' Explanation: String has 4 or fewer characters, return unchanged Input: cardNumber = null Output: null Explanation: Null input returns null Input: cardNumber = '' Output: '' Explanation: Empty string returns empty string
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