Write the smallest 4-digit number that can be formed by the digits 0, 1, 3 and 6, using each digit only once.

The first digit of a 4-digit number cannot be 0. So, the smallest available non-zero digit is 1.

Thousands place: 1

The remaining digits are 0, 3, and 6. To make the number as small as possible, arrange these remaining digits in ascending order: 0, 3, 6.

Hundreds place: 0

Tens place: 3

Units place: 6

Combining all the data, the smallest 4-digit number is 1036.

Hence, the smallest 4-digit number is 1036.