At first, you should have your Windows installed in APCI mode. If you install your OS in PnP/APM mode, you have to enable your ACPI support in BIOS setup menu.

_S1: CPU stop clock
_S3: power off system except memory and wake up cricuit
above sleep call suspend(待命)
OS decide which sleep would go to, depend on driver and hardware status.

_S4: hybernation(休眠), save memory map to DISK file and shutdown, When next time starp up system, the memory map is totally restorted.

You can go to _S4 by press:
Windowkey then U then alt-H

you can goto 電源選項 to setup power manangment, or just press power button.