To enable 4GB RAM in 32 bit Windows OS, you can use PAE, but it’s not nearly as good as just going 64 bit.
For more information, you can check this MSDN Page:
Physical Address Extension
http://msdn.microsoft.com/en-us/library/aa366796%28VS.85%29.aspx
Memory Limits for Windows Releases
http://msdn.microsoft.com/en-us/library/aa366778(v=VS.85).aspx