How to get xp pro to recognize 4gb ram

It didn't, it won't, it can't. Give up. 3.25GB is the absolute max. Thank M$ for that.
Remember, the first IBM PC could only address 504KB of RAM (rather than all 640KB) because it was believed nobody would ever use more memory than that.

 

lewza

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You will find you are using a 32bit version of XP, this will not recognise no more than 3.5gb of memory? No matter if you have 4gb or 8gb.

If you upgrade to 64bit then you can utilize as much as your motherboard can handle.
 
On the other hand, you can create a RAMdisk from the remaining memory. It's a kluge, but there it is. Put your pagefile on it, and you'll page pretty quickly.'

To answer a question that you may ask next, you cannot upgrade from 32-bit to 64-bit version. It requires a full install.