When people say matched pairs (as in "dual-channel"), they mean two sticks of RAM that are of the same exact type. In your case, it probably just makes more sense to go for the 4GB module. You *might* get slightly better memory PERFORMANCE with the 4GB, but the extra physical capacity in 6GB greatly outweighs that.
You can buy from Crucial, but OEM might be cheaper anyway. It doesn't make that much of a difference, as long as you get a module that is DDR3-1333 PC3-10600.
If you want more memory with better performance, you might consider just going for it and getting a 2x4GB pack. Obviously it's your choice though.