You can try, there's no guarantees when mixing DRAM. Briefly dual channel is when you have a like (same) amount of DRAM in each channel and the MC (memory controller) treats it as a single 128bit device rather than 2 separate 64 bit devices (as each stick is an individual 64bit device). In your case here with a 4 and a 2GB stick, if they are in slots 1-3 or 2-4 then they will be running in whats called Flex mode 2GB of DRAM from each channel is running dual channel and the odd 2 GB from the 4GB stick is in single channel mode.
More on Flex in my article FAQs, item 7, here:
http://www.tomshardware.co.uk/ddr3-dram-faq,review-33220.html
If you add an 8GB and it will play nice, would put the sticks from the CPU as 4GB - 2GB - 8GB - empty. In Flex this will put 12GB in dual and the odd 2GB in single