Which format specifier is used for printing an unsigned integer in hexadecimal with at least 4 characters?

The format specifier used for printing an unsigned integer in hexadecimal with at least 4 characters is "%04x".

Here is how it works:

1. The "%" symbol is a placeholder that indicates a format specifier is coming up.
2. The "0" is a flag that indicates zero-padding should be used. This means that if the number to be printed has fewer than 4 digits, zeros should be added to the left until it reaches 4 digits.
3. The "4" is a width specifier that indicates the minimum number of characters to be printed. If the number to be printed has more than 4 digits, all of them will be printed.
4. The "x" is a conversion specifier that indicates an unsigned integer should be printed in hexadecimal format.

So, if you have the number 10, for example, and you use the "%04x" format specifier to print it, it will be printed as "000a", because 10 in hexadecimal is "a", and the zero-padding adds three zeros to the left to reach 4 characters.