Why can't a 32bit OS access 4GB of memory? Because 2^32 = 4,294,967,296 bytes = 4096 MB. Now this address space is shared by RAM as well any memory mapped I/O devices (all devices'/ports' haf an address assigned to them for communication which shares this address space). Hence some part of it (e.g 0.25GB) is reserved for them.
What is PAE? PAE increases the 32-bit limit to 36-bit provided the CPU is 64-bit. Theoretically thats 2^36 bytes = 64 GB, but some OS' haf a limitation on the addressing limit.
You can force physical address extension on 32bit OS'. To enable PAE, start Command Prompt in admin mode and type this: BCDedit /set PAE forceenable
You will need to restart your system. This may or may not work for you. To disable PAE, replace 'forceenable' with 'forcedisable'.
If it works and you've enabled PAE you may find your system to be a bit slow. This is due to the extra addressing overhead. You may want to turn Data Execution Prevention (thats a hardware method of prevent buffer overruns). To do that, open command prompt in administrator mode and type these series of commands:
1) BCDedit /set nx AlwaysOff
(This turns off DEP alongwith PAE)
2) BCDedit /set pae ForceEnable
(Force PAE to turn on, inspite of disabling DEP)
Get back to us and let us know if it works for you and about the performance.