4GB of DDR3 RAM is substantial. Yes, you would notice a difference, but only with certain applications (e.g. CS4). If you're typing essays and watching youtube, for instance, you won't notice a difference.
Personally, I would suggest upgrading through Apple. Presently, DDR3 memory is quite expensive and for once Apple's memory is aggressively priced. The cost to upgrade with Apple will cost about the same as a 4GB kit from a 3rd party vendor (give or take $10).
Of course, if you upgrade with Apple, you lose the 2GB that came with the computer. If you upgrade with a 3rd party, you have to install the RAM yourself, but you can sell the extra RAM that came with the computer.
HOWEVER... there have ben issues with installing 3rd party DDR3 RAM in the new MB/MBPs, so if you're going to go the 3rd party route, I suggest you wait a month or two until the wrinkles are ironed out.
Hope that helps!