Explanation:

It is given that the first digit is 1/3 of the second. There are 3 such possibilities
1)1 and 3
2)2 and 6
3)3 and 9
Now, the third digit is the sum of the first and second digits.
1)1 + 3 = 4
2)2 + 6 = 8
3)3 + 9 = 12
It is clear that option 3 is not possible. So we are left with only two options.
Also, the last digit is three times the second, which rules out the second option.
Hence, the answer is 1349.