I'd likely suggest the 2x2 GB route for dual channel vs. single channel, not to mention i imagine you should be able to find decent 2x2 GB kits for a bit less.
I don't really like suggesting buying single DIMMs as it's unlikely you'll be able to buy an identical DIMM in the future.
Not that it's the end of the world to run mismatched RAM, but i'd always try to avoid that scenario to be safe.
If you do decide to go to 8 GB later on, i'd then say sell the 2x2 GB kit, & purchase a 2x4 GB kit.
That's the way i'd always go.