4. (8 points) The units digit of 2010 2009s is $\qquad$

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【Solution】Solution: The unit digit is determined only by the unit digit of a multi-digit number, that is, the unit digit is determined by the product of 2010 nines,

According to the cycle of $9, 1$ every 2 digits,
$$
2010 \div 2=1005
$$

There is no remainder,
Therefore, the unit digit of the product of 2010 nines is 1;
The answer is: 1.