where also is fourth wire hooked in?
your 4 wires are probably the white, black, ground, and traveler wire. The 4th wire, the traveler wire is needed if you want to have a 3-way or 4-way switch. Since you don't have this type of switch, otherwise it will be obvious where the 4th wire goes, just put a cap on it and tuck it inside the box.
If it is coming from the box at the street into your circuit breaker panel, I'll take a guess. Earth ground (Usually connected to a copper water pipe or a metal pole driven deep into the ground) Neutral (0V) +120 V (Most household circuits) +220 V (Dryers, stoves, possibly air conditioners and other devices needing lots of power. These have special plugs.) Unless you are ABSOLUTELY sure you know what you are doing (and it sounds from your question like you may not be) DO NOT mess around with mains wiring in your house and especially wiring coming into your house, since that isn't even on a surge protector to keep you even remotely safe. If you mess around with this stuff without knowing what you are doing, you could wind up DEAD or in the hospital VERY easily. I can't stress enough how dangerous electricity in large quantities is.
If you really have a 3-pole 4 wire system, the 4th wire is the neutral. (The other 3 wires are the phase wires.) It is much more common to have 2 phases with a neutral and a ground for a house. In either case the 4th wire connects to the neutral bus. How do you know which wire is the 4th wire? Test the voltages between the wires. You will have a bolt of lightning, if you connect the wrong wire to the neutral. Luckily, touching the incoming wires to your house is very rarely lethal.
