Your MacBook accepts up to 8GB of RAM. See here for info on upgrading it: http://support.apple.com/kb/HT1651
You did read that right. Apple lists what they think should be the maximum amount of RAM. However it is not correct. Your Mac can accept 8GB of RAM. Take a look at OWC (just click on OWC) that will take you to the site that sells RAM specifically for Macs. Your Uni MacBook can accept up to 8GB of RAM 2X4GB. Take my MacBook Pro for instants, Apple claims the maximum amount of memory is 8GB. But in reality it can take up to 16GB. It has more to do with the Processor, rather than the logic board itself. You can take up to 8GB.