Is my wiring sufficient to change to a WiFi thermostat for my boiler?

[BobbyK]

I have a single stage oil boiler, it pumps hot water to four zones, and is controlled by an Argo relay control box. All my t-stats are standard Honeywell T87F's (the circular, manual t-stats w/ mercury). I would like to replace one of the Honeywells with a WiFI t-stat, and the Ecobee Smart SI Wifi t-stat looks like a good choice to me.
Links to direct purchase sites aren't permitted

I'd like to ask if my existing wiring is sufficient to connect the Ecobee. My current t-stat wire has 3 leads: red, white and green. The red and white wires are connected on both ends, but the green wire is not in use. I know that the Ecobee needs 24VAC in addition to the normal wiring. Is having one spare wire enough to hook up the 24VAC or do I need two spare wires? (4 wires in total)

Also I'd like to ask exactly which terminals should be connected. I've attached some diagrams to help make this clear. FYI I'm an audio guy and I do audio wiring but electrical wiring is not so familiar to me.

Currently the white and red wires connect from the T-T terminals of zone 4 on the Argo box to the Honeywell t-stat.

My confusion lies in how to connect the 24VAC. The Ecobee has a terminal for Common, but no terminal for 24VAC. (see Ecobee attachment)
On my Ago box attachment, you can see on the right side there are two terminals, one above the other: the top one is labeled Common, and the bottom one is labeled 24VAC. I'm not sure if I should connect the Common terminal on the Argo box to the Common terminal on the Ecobee? What is the Argo's 24VAC terminal used for?

I asked a contractor about this and he said "it's better to have a 4-wire connection, I should come and run another wire to the Argo". I just don't know if I NEED to have someone run a 4th wire for this hookup - or can I do it myself with my existing three wires??

Thank you all in advance!

[54regcab]

I'm thinking you may have some compatibility issues since your current system has the heat/cool switch somewhere other than the thermostat. Your current stats sound like "heat only" or special zone control stats. Your zone control board may need to be updated for the ecobee to be compatible.

[BobbyK]

Hi 54regcab -
Sorry, not sure what you mean by this

I'm thinking you may have some compatibility issues since your current system has the heat/cool switch somewhere other than the thermostat.

I don't use cooling. Just a single boiler for heat. Can you explain a bit more please?

[54regcab]

Even heat only wiring could get fun with that old zone control. You need to find somebody farmiliar with the older zone controls.

[kangaroogod]

3 wires will work just fine

[pacnw]

why do you want to change the stat?

if strictly to have internet access, okay.

if any other reason, then most likely not going to accomplish it.