Do not turn it off. Some apps will just not deal well with it, and others may allocate enough memory to want a bit of swap even with 4GB. The imaginary boost in performance just isn't worth potential hassles it may bring by being off.

Looks like Windows 7 set mine to 2GB, or possibly I set it manually, can't remember. This is on a system with 14GB of total RAM.

On the OS X side, the system has allocated 128MB to swap space on it's own, even though I still have 6.91 GB of completely idle RAM.