How many four-digit, positive integers are there where each digit is a prime number?
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
How many four-digit, positive integers are there where each digit is a prime number?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
256
How many four-digit, positive integers are there where each digit is a prime number?
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