You can use all 4gb of RAM in Windows XP without installing the 64bit version. If you have all the RAM installed when installing Windows XP, it should recognize and use all of it without manipulation. If you installed Windows XP with less than 4gb of RAM and then upgraded, you will need to use the /pae switch on your boot.ini file. Here is more detailed information:
http://www.microsoft.com/whdc/system...AE/PAEdrv.mspx