How find out what the second switch button is for in bathroom

If I understand your layout, you could wire it like this:

Chaining to all four lights like this requires the use of 12/3 cable for the first two sections, but if you want to follow Tester101's suggestion and add the extra conductor to the fan circuit for possible future use, you're going to buy some 12/3 anyway.

EDIT: Carl Witthoft is correct, the bathroom should be protected by a GFCI. I have altered my diagram to show a GFCI outlet with downstream protection.

OP seems to have done a scarper but for anyone else reading this:

The one sure way to find where the power enters a circuit is to completely disconnect each cable and drop a test load across its wires.

I detach the cables one at a time and re-connect each one before going on to the next. This saves having to mark everything first -- although it does involve a lot of running back and forth to the circuit breaker panel.

My favorite test load is a neon indicator lamp on a short piece of lamp cord with two insulated alligator clips. But almost any kind of lamp can be pressed into service.