4 is the max that XP and 32bit Vista will recognize.
However, they can't actually use all of it, generally speaking. It's so cheap, that I would just get the 4GB and not worry about it, though.
I have both a 6 cell and 9 cell for my 1525. I actually have the 6 cell in most of the time, but I have it plugged in most of the time too. I highly recommend the 9 cell for traveling, but it does stick out the back of my laptop a bit so it's a bit more unwieldy, and it's noticeably heavier with it installed.
I wouldn't go with the 4 cell unless you plan on having it plugged in a lot, or will be using it for less than roughly an hour at a time unplugged.

