You use an if expression based on this field.
Let us say the field is called 'Quarter'
You can use this as an expression
=If ( Quarter = 3, X,
if( Quarter = 4, Y)) )
where X and Y can be any expresion.
This is will only work if only one value in the list box is selected. To achieve it, you can go to the list box properties, general tab and check 'one selected value' ( you need to select one value there first)
Hope this helps,