You're not saying the months are listbox columns (i.e. 4 months, 4 columns)? If so, that's not good. If they are list items and a user is picking from the list, you need to iterate over the selected items and build an expression with + in between the items. That assumes the items are field references or numbers. If you search 'loop over listbox selected items' you'll find lots of code examples.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.