What is the wire labeled '4' for? Seems to work without it

[merk]

Hi all,

Moved into an apartment and I wanted to swap out the old dumb thermostat for a nest thermostat. Below photo is the old thermostat. It had 5 wires labeled G, RC, Y, W and 4. When I go the nest site to check if its compatible, it says it's not because of the '4' wire. I decided to just try hooking it up anyhow and ignoring the 4 wire. And as far as I can tell everything is working. Heat/AC both will turn on. I can turn on just the fan with no heat or A/C.

But just because it seems to work I figured I should ask here first before I actually start using the nest thermostat. Can someone tell me what that 4th wire was supposed to do? Is it a dud if everything seems to work without it? Let me know if you need additional info.

Thank you

 

[user 4883]

Are you referring to the red jumper wire coming off RC?
I'm not familiar with Nest wiring but, chances are you don't need it with the nest.

 

[merk]

Yes, that's the wire I'm talking about.