Yes, you can. As long as you are installing as a pair, you can replace two 2GB (2x2) with two 4GB (2x4).
This guide was great. I added a few comments under specific steps but my few tips are: (1) Use ifixit’s project tray or magnetic mat to hold all of the screws; (2) it’s an opportunity to upgrade the hard drive at the same time so plan out both by reviewing the hard drive upgrade guide as well; (3) have patience; (4) I spread out across my dining room table and pretty much used most of it so have a big surface area for this!
I found this connector to be very tight and was afraid of breaking it off. I just kept slowly working on it—it did eventually come out.
I did try to sneak the RAM in without removing the logic board but my fingers were too big. I found it too difficult and just continued on with this guide.
I would recommend that those trying this have the project tray to contain all of the screws.