You won't blow anything up. They have slightly different voltage requirements (1.5V vs 1.6V), but that may not be an issue. The memory controller will default to the lower voltage. The timings aren't at all alike, so the memory controller will probably default to the slower timings. It's always best to use matched sets of RAM.
It should be fine, it'll just default to the slowest timings of the bunch and lower voltage, worst case scenario you'll need a bit of tweaking to get them stable. Also make sure you place both 2gb sticks and both 4gb sticks on slots of the same color, for example, if your motherboard has 2 blue slots and 2 black slots place both 2gb sticks on the blue ones and both 4gb sticks on the black ones, otherwise dual channel may not work.