What is the smallest number that can be formed by the digits 4, 7, 1, and 9 such that the digits are not repeated? 

The given digits are 4, 7, 1, and 9. 

Now to obtain the smallest number, we need to arrange digits in increasing order.

Therefore, 1 < 4 < 7 < 9. 

So, the smallest number that can be formed by the digits 4, 7, 1, and 9 such that the digits are not repeated, will have 1 in the thousands place, 4 in the hundreds place, 7 in the tens place, and 9 in the one's place. 

Hence, the smallest number is 1479.