4. (5 points) The digits $1, 2, 3, 4$ can form many four-digit numbers. Arrange them in ascending order, 4123 is the $\qquad$th.
【Answer】Solution: Starting with 1, we can get 6 four-digit numbers, which are $1234, 1243, 1324, 1342, 1423$, and 1432. The same applies when starting with each digit. Similarly, starting with $1, 2, 3$ we can get a total of 18 numbers. 4123 is the first three-digit number starting with 4, which means 4123 is the 19th number. Therefore, the answer is 19.