You may be able to use Physical Address Extension (PAE) to address more than 4GB. PAE increases the address size from 32-bits to 36-bits or more so you could use 64GB or more.
PAE is configured in your system BIOS so it can make a difference for any operating system including Linux and Apple OS X.
Windows 32-bit uses PAE but locks in the 4GB limit. There are utilities that remove this limit but I don't recommend using them as Pushing the Limits of Windows: Physical Memory says.