4. How many four-digit numbers are there for which the sum of the digits is a multiple of ten?
4. Answer: 900.

To any three-digit number, you can append exactly one decimal digit on the right so that the sum of the digits becomes a multiple of 10. For example, to 780, we append 5; to 202, we append 6; to 334, we append 0, and so on. Since three-digit numbers range from 100 to 999 inclusive, there are exactly $999-(100-1)=900$ of them. There will be the same number of four-digit numbers that meet the condition of the problem.