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The digits are 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 - in all, there are 10 digits.
Any of 10 digit can be placed on the most-left position. It gives 10 options.
Any of 10 digit can be placed on the next position. It gives 10 options.
Any of 10 digit can be placed on the third position. It gives 10 options.
Any of 10 digit can be placed on the fourth position. It gives 10 options.
All these options are INDEPENDENT. You can combine them in ANY way.
Therefore, there are possible combinations of 10 digits in 4-digit words.
It is the explanation you wanted.
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It is THE STANDARD explanation/solution for such problems.
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By the way, this fact is STRICTLY PARALLEL to the fact that there are integer numbers from 0 = 0000 to 9999 inclusively.