If you believe 4GB ram prices will continue to drop then it might actually be cheaper to upgrade with 2x2 now and replace them all with 4GB rams later.
For instance, when I bought my iMac last November, 2x4GB was over $400 but 2x2GB was $110 (this at Crucial.com). I upgraded to 8GB with the 2x2 at the time, and if I wanted to go to 16GB now it would be $600 (4x4GB) for a total of $710. If I bought 2x4GB then and bought two more now to get 16GB, I would end up paying over $700.
Of course I'd have the benefit of 12GB instead of 8GB for the past 9 months, but I've never maxed out the 8GB yet.