Do not fill in both set of fields

I have two set of fields, fields A, B, C, and D and fields 1, 2, 3, and 4.

Total two sets of fields, each with four fields of their own.

If user choose to fill in alphabets then they are not allow to fill in numbers.

If user choose to fill in numbers then they are not allow to fill in alphabets.

So far my solution is either use drop down list or radio button to group these two set of fields, but it's an extra step for user to pick option from radio button\drop down list.

What's the best way to present this to user? So that at first glance, they know exactly what to do.