If you are running more than 2GB of memory and are using FSX-SP2, disable hidden files and folder, then on the root C:\ you will find a file named: boot.ini
Open it in notepad and add the follwoing to the end of the OS boot line:
/3GB/Userva=2560
multi(0)disk(0)rdisk(0)partition(2)\WINNT="Microsoft Windows XP Professsional" /fastdetect /3GB /Userva=2560
NOTE: The Userva= value should ONLY be tweaked based on values which are divisible evenly by 64
This will allow you to use more than 2 gigs of your 4 but you are never going to get 4gigs to work without a 64bit OS, more like 2.6
WARNING: setting that higher than 2560 will most likely crash the computer. Windows needs a reserve address space for the OS and video card memory mapping
Here is the MS KB readup on the line and its use
http://support.microsoft.com/?kbid=316739