15. (10 points) (1) Among the 3998 natural numbers from 1 to 3998, how many are divisible by 4?
(2) Among the 3998 natural numbers from 1 to 3998, how many have a sum of digits that is divisible by 4?
【Solution】Solution: (1) $3998 \div 4=999 \cdots 2$ indicates that there are 999 multiples of 4 from 1 to 3998, so among these 3998 natural numbers from 1 to 3998, there are 999 numbers that can be divided by 4.
(2) Classify the natural numbers from 1 to 999 according to the remainder when the sum of their digits is divided by 4, (1000, 2000, 3000 do not meet the conditions and are not considered) First category: 1, 5, 9, 10997 (remainder 1 when divided by 4)

Fourth category: $4, 8, 13, 17 \cdots 996$ (remainder 0 when divided by 4)
Among the above four categories, only the fourth category can be divided by 4. If 3 is added to the thousand's place of the first category, 2 is added to the thousand's place of the second category, and 1 is added to the thousand's place of the third category, the sum of the digits in the first, second, and third categories can all be divided by 4. It can be seen that, among the 3998 natural numbers from 1 to 3998, only these numbers meet the condition, so there are
999 numbers from 1 to 3998 whose sum of digits can be divided by 4.