<pre>
Leave switch 1 off all the time.
Turn switch 2 on for a couple of minutes, then turn it off.
Turn switch 3 on and leave it on.
Now, go to the bulb.
If the lightbulb is on, switch 3 controls it.
Otherwise, if the lightbulb is hot to the touch, switch 2 controls it.
Otherwise, switch 1 controls it.

Since incandescent bulbs do not get hot immediately, it is possible to
distinguish between four switches.  The trick is to turn switch 4 on
just before you go into the other room.  If the lightbulb is on and
hot, than switch 3 controls it, but if it is only warm, then switch 4
controls it.
</pre>
