Note: There are many ways to solve this problem. One way is to start by finding the second and third digit of the combination. As the problem stated, the first two digits are the same and adding them together produces the third digit. That means the third digit is actually twice as much as the second digit. We also know that the second and third digit form a perfect square whose square root is the fourth digit. Looking at the two-digit perfect roots 16, 25, 36, 49, 64, and 81, we see that there's only one that has a third digit that's twice as much as the second digit: 36. That means the first digit must also be a 3. Adding 3 and 3 does give 6. Now, we take the square root of 36 we get 6 so the fourth digit is also 6, which is the same as the third digit.
This means the combination is 3366.