For a number to be divisible by 4, the number formed by its last two digits should be divisible by 4.

∴ possible last two digits here are 12, 24, 32

By taking 12, possible numbers = 3412 and 4312

By taking 24, possible numbers = 1324 and 3124

By taking 32, possible numbers = 4132 and 1432

Smallest number among above = 1324

Hence, using each of the digits 1, 2, 3 and 4 only once, the smallest 4 - digit number divisible by 4 = 1324