$4.1 \times 1+2 \times 2+3 \times 3+\ldots .2011 \times 2011+2012 \times 2012$ The last digit of the sum is $\qquad$
【Answer】0
【Analysis】Key point: unit digit, sequence pattern
The unit digit of each term in the expression cycles through the ten numbers $1, 4, 9, 6, 5, 6, 9, 4, 1, 0$. $2012 \div 10=201 \cdots 2$,
The unit digit of the expression is equal to the unit digit of $201 \times(1+4+6+9+5+9+6+4+1)+1+4$, which is 0.