Even though I bought both at the same time (my Mac Mini & RAM), I waited a few weeks before I upgraded my RAM.
I felt a BIG improvement when I did the upgrade. I only did this to see if it was worth it. I wanted to get used to the "regular" speed before I changed the RAM out. If having the speed isn't that critical for what you do, wait it out and see if you actually notice the difference when you change it.
At least its easier in the MBP than what I had to do to change out RAM in my Mini. Changing RAM out in the MBP is a breeze. Try stock for a week or so. If you cant feel a difference, then you may not need it. Then again, If you already bought it and installed it, just enjoy it and impress your friends telling them about your 4GBs of RAM. Some could only wish.