I assume you already know how to call an air support. If you don't, afraid someone else will have to answer you -haven't touched OFP for a long time and now I'm a little lost, so I only can remember some very basic things-. Ok, you must name them. For example, shilka1, shilka2, etc.
Now, create a trigger. Put in its condition:
!(alive shilka1) AND !(alive shilka2) AND !(alive shilka3) AND !(alive shilka4)
on activation:
1 setradiomsg "Call air support"
Now go to your mission's folder, create a file called init.sqs, and put in it:
1 setradiomsg "NULL"
You must have created before trying it a trigger that is activated by Radio Alpha with a condition that must be that which will make your air support work.