Step 1: Understand the Divisibility Rule for 8

A number is divisible by 8 if the number formed by its last three digits is divisible by 8.

$4a6 \text{ must be divisible by } 8$

Step 2: Test values for $a$ starting from the least digit (0)

Let us test $a = 0$: The three-digit number is $406$. Dividing $406$ by $8$ gives $50.75$ (not an integer).

$406 \div 8 = 50.75$

Step 3: Test the next value $a = 1$

Let us test $a = 1$: The three-digit number is $416$. Dividing $416$ by $8$ gives exactly $52$.

$416 \div 8 = 52$

Step 4: Conclusion

Since $416$ is divisible by $8$, the least possible digit for $a$ is $1$.

$a = 1$