How many wires do I need for ecobee4 to work with a boiler?

For ecobee to work with a boiler, it requires the R, W and C wires. If you do not have these wires, contact your ecobee preferred contractor and ask them to run the missing wire(s) for your ecobee4 to support a boiler.

You can also consider using a Fast Stat Common-Maker or installing an isolation relay but not all systems require an isolation relay). The best way to determine this is to call your contractor.