Answer: A number four-digit is a four-digit number. A 4-digit number’s first digit should be 1 or larger, while the subsequent digits can be any integer between 0 and 9. The first four digits of a four-digit number are 1000 and the last four digits are 9999. Ones, tens, hundreds, and thousands are the place values in a four-digit number, starting from the right.
As a result, the total number of 4-digit numbers equals (9999-1000)+1 = 8999+1 = 9000.