The largest 4-digit number with two different digits is 9998. Its successor is the next number, which is 9999. However, 9999 has only one digit (9), so we need to find the next valid 4-digit number with two different digits. This number is 10000, which is technically a 5-digit number, so the largest valid number with two different digits before 9999 is 9998, and its successor cannot be a 4-digit number with two different digits. Thus, there is no valid successor that meets the criteria.
9876543
987,654
1234567
9998888
99,999,876
9876543
9999876
987,654
1234567
9998888
10000023
102,345
It is 1,000,002
99,999,876
99,999,987
999987
10002