(1) According to the meaning of the question: a =1-(5%+105+15%+15%+30%) = 25%,

The total number of eighth-grade students is 2010% = 200 (people);

(2) The number of active people in five days is 200×25%=50 (people), and the number of active people in seven days is 200×5%= 10 (people).

Complete the statistical chart, as shown in the figure:

(3) The mode is 4 and the median is 4;

(4) According to the meaning of the question: 6000× (30%+25%+15%+5%) = 4500 (person),

Then there are about 4500 people who are active for no less than 4 days.