The pagefile/swap is used to add performance and free up ram (which is quicker in access) to priority tasks.
So none priority tasks is moved to swap to free up ram for tasks with higher priority. It's pretty much always used. It is recommended to leave pagefile settings to original and let windows increase or decrease it as it sees fit.
If i remember correctly windows will keep a ratio of around ram * 1.5, but might have changed with newer versions of windows but I think it's still a good rule
So 4 gb ram should have a pagefile of around 6 gb (unless windows itself has deemed it fit with less
)
EDIT:
Here we go, seems to still be recommended with ram * 1.5. Or actually let windows manage it as always
http://technet.microsoft.com/en-us/m.../ff382717.aspx