What low-voltage thermostats are compatible with Integration Kit?
Third-Party Low-Voltage Thermostats:
Nest 1st Generation
Nest 2nd Generation
Radio Thermostat CT80 (see http://www.radiothermostat.com/wifi/)
Radio Thermostat CT50 (see http://www.radiothermostat.com/wifi/)
Crestron CHV-TSTATEX - infiNET EX® Thermostat (TERMINAL FOR FLOOR SENSOR IS AVAILABLE)
Control 4 Control4® Wireless Thermostat CCZ-T1-W (TERMINAL FOR FLOOR SENSOR IS AVAILABLE)
Not compatible based on the specs, but have been tested and working:
Honeywell Wi-Fi VisionPRO® 8000 (TERMINAL FOR FLOOR SENSOR IS AVAILABLE)
Savant Wireless Thermostat SST-W100 (TERMINAL FOR FLOOR SENSOR IS AVAILABLE)
RC840T or Integration Kit Transformer Rating is 24 VAC / 40 mA / 1.2 VA, so any low-voltage thermostat that is compatible with this voltage and power will work with RC840T, if the power consumption (draw) is higher, then additional transformer is needed. Here's an example: Honeywell's Transformer AT120B1028 that provides 24VAC / 20VA
Ecobee 4 would not be able to run floor heat as an independent heating source or zone, it can only run it as a 2nd stage heat. https://support.ecobee.com/hc/en-us/articles/360002020552-Threshold-settings-for-ecobee-thermostats-ecobee3-ecobee3-lite-ecobee4-
Also, no floor sensor on Ecobee 4, sounds like not the best way to control floor heat.