If 4GB is soldered onto the motherboard, and it comes with a single RAM slot with a 2GB stick, is that dual channel? If not, would it be if the 2GB stick was replaced with a 4GB stick? What about 8GB? Or does a single slot mean no dual channel?
Both the motherboard and the CPU have to support dual channel for it to work, and at minimum the sticks will need to be the same capacity. There's more to it than that but you didn't really give us enough info to help you out. Personally though I wouldn't expect dual channel, I've on seen it on gaming laptops and even then it doesn't make a huge difference.
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In this case it works with a mixed dual channel, were the first 2 gb of the solder in memory work in dual channel with the 2 gb of the stick, then the remaining 2 gb of the solder memory work in single channel, motherboards have done this since pentium 4 times.