If you allow the combination comprising no digits, then 2^16 = 65536.
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2 to the power 16.
If you use them only once each, you can make 15 combinations. 1 with all four digits, 4 with 3 digits, 6 with 2 digits, and 4 with 1 digit. There is also a combination containing no digits making 16 = 24 combinations from 4 elements.
A quadrillion has 16 digits.
16 digits.
there are 16 digits on a visa card