That's good. Since you have the retail version, you qualify to get the 64-bit version for the price of the DVD media plus S&H. It comes to about $10 and uses your existing Vista product key/license. So, if you're really concerned about not getting all 4 gigs, you can opt for 64-bit. If you don't like 64-bit, you can stick with 32-bit and accept the missing RAM.
Go here for details:http://www.microsoft.com/windowsvista/2057/ordermedia/default.mspx