If you can get wires there, you can average 4 (or more) sensors together. Otherwise, switch to Lutron and you can use up to 5 wireless sensors per repeater and average them. You'll have to add a Ra2 main repeater, but would also gain the ability to use Lutron Occ/Vac sensors, and visor controls.

If you really want to use the C4 thermostat and retrofitting wires isn't possible, you might be able to use HL software thermostat driver paired to CardAccess devices and then use the Master Thermostat driver to combine/average them. The software thermostats would just be dummies for temp feedback to the master driver

[Link: houselogix.com]
[Link: houselogix.com]