I think I know but you mean but not positive.
In your simple example all answers are correct so just count-selected(${previous_question}) would work.
But if you have to test each one then all I can think of off the top of my head is to have 4 fields.
if(selected(${field1}, "Correct answer"), 1, 0)
if(selected(${field1}, "Correct answer 2"), 1, 0)
etc. Then add those 4 fields up. Of course this can get really long depending on the list.
Hope that helps.