I just purchased an ecobee4 and I found that I need C wire for wifi which is not connected to my current thermostat. However, there 2 extra wires I found in the wall. Could you use one of them for the C wire? Also the board has something already going in to C screw but I think its for something else? It's coming out of the separate brown cable. There is also one wire coming from brown cable going to yellow. I leave these 2 alone?
Go ahead, hook that blue wire up to C and use it for your Ecobee
As the terminal strip on your furnace control board is typical in that it has room for 2 wires on each screw, you can simply land the blue wire on the furnace control board's C terminal alongside the white wire that's already there.
BTW: the brown cable you're asking about can simply be left alone -- it's the cable that connects your thermostat's yellow wire (Y terminal) and the furnace control board's C terminal to the compressor contactor in your air conditioner so the thermostat can turn it on and off when things get sweltering in the summertime.