Question 749500
You have two choices for the tens and ones digits, respectively.  

The number will be xx13 or xx39, the sum of the first two digits is the same as the last, and the sum of all four is 21.  If the sum of the first two digits was 3, the sum of all four would only be 7.  If the sum of the first two digits is 9, then 9+3+9=21.  Therefore the number is of the form xx39 where the first two digits add up to 9.

9039
1839
8139
2739
7239
3639
6339
4539
5439

Those would all work