How do I limit the number of times a radio button can be selected?

I am looking to create an appointment time sign up and need to limit the time slot selected to only be chosen 4 times, and then no longer available.

We actually have a widget made specifically for appointment time slots.  It is our appointment slots widget:

http://widgets.jotform.com/widget/appointment_slots

You can set the amount of each time available and when a selection is made, the counter will decrease.  When the counter reaches 0, the option will be disabled.