What is the largest 4 digit number?

The biggest number with 4 digits is 9999.

It is a simple logic. See, the largest single digit is 9. A 4-digit number has four places. To make the number as big as possible, we should fill all places with the biggest digit. So we put 9 in the thousands place, 9 in the hundreds place, and 9 in the other two places also. This makes 9999. If you add just one to it, you get 10000. And that number is having 5 digits. So 9999 is the final answer. I hope this is helping.