Connecting a thermostat to air handler and cast iron boiler
If you originally had 4 thermostats, each controlling 1 zone, then the normal way for wiring was that a thermostat opened and closed each zone valve and when the zone valve opened an end switch made and allowed the boiler to fire. Zone valves can be either 2 wire or 3 wire hook-up. Most are 2 wire but there are some 3 wire available. AS I said, that is the normal way. How yours were hooked up is anybodies guess. Now for the air-conditioning system; one of the thermostats could control the A/C, or there is a separate thermostat for that. Does the air handler have a hot water coil mounted in it or is it just for air-conditioning? Check each thermostat you removed and determine how many wires were hooked to each thermostat. You may have to trace all the wiring to see where the wiring went and how the connections were made. I would suggest that you consult an HVAC company in your area that has a good reputation to help in case this is above your area of expertise